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Posted: 12/20/2014 3:47:24 PM EST
I'm hoping an Engineer or somebody else with a good grasp of physics can chime in on this.

I bought a Low Mass BCG because I wanted an AR with the least amount of recoil possible. I heard that heavy BCGs are better because they move slower making less recoil, but I also heard because there's more weight then there's more recoil because of more mass moving. I ended up buying the latter point of view, but now I'm thinking I'm an idiot.

What I can remember from Physics is that it shouldn't matter one way or another if it's a light or a heavy bcg. The amount of energy pushing on the BCG is the same, so so a lighter BCG just moves faster and a heavier BCG just moves slower and either way the momentum of the light or heavy BCG are virtually equally meaning the recoil hasn't changed one bit.

As part of my recoil strategy, I got an extra stength buffer spring, hydraulic buffer, and a tuneable gas block. The idea is that with the extra strength of the spring and tuning the gas just right, I can get it so that the buffer never really hits the back of receiver extension, and if it does, the hydraulic buffer will dampen that last bit of movement. I was aiming for the constant recoil principal of the AA12 shotgun and Ultimax 100 machine gun where the BCG never hits the rear and is fully arrested by the springs. In automatic fire, a slower rate of fire reduces recoil beyond the constant recoil principal because you can average the energy of all those shots in rapid recession. But in a semi-automatic where you're only shooting one at a time, that wouldn't make a difference.

So, the question is, could I have just as well went with a heavy BCG? Wouldn't the stronger springs and tuned down gas react the same to a heavier/slower bcg the way they would with a lighter/faster bcg? In your answer, let me know if you're an engineer or similar that has a background understanding the math and formulas behind all this stuff.
Link Posted: 12/20/2014 4:03:58 PM EST
I think the secret is a light BCG and a adjustable gas block, to slow it to normal BCG speeds
Link Posted: 12/20/2014 4:06:29 PM EST
I'm a 3 gunner, so here's "our" collective process for recoil reduction.

Empty the weights out of the buffer, get a lighter action spring, get a low mass bcg, handload some reduced power loads, dial the gas down to where your bolt just catches with the reduced power reloads and turn it another 1/2 turn open.

In addition to recoil reduction, I would recommend getting an effective brake/comp. A DPMS Miculek muzzle device is about half the price of the big dogs and delivers about 80% of the recoil reduction and actually does a better job in reducing muzzle flip than some of the big names.
Link Posted: 12/20/2014 4:06:52 PM EST
[Last Edit: 12/20/2014 4:09:22 PM EST by Essayons]
The recoil impulse that a low mass carrier and an adjustable gas system minimizes is the one that occurs when the bolt carrier group and buffer strike the back of the receiver extension (the "buffer tube"). You're minimizing the reciprocating mass and adjusting the system so it has just enough gas to cycle reliably with the ammo you intend to use. That minimizes the energy imparted by the reciprocating parts on the receiver extension. On the other hand, you are giving up reserve power by reducing the mass and momentum of the reciprocating parts. That makes the weapon more sensitive to variations in ammunition and less reliable in adverse conditions. As a result, you see low mass operating systems most often on competition or recreational rigs.

Low mass operating systems also minimize the mass reciprocating around the weapon's center of mass, which arguably minimizes the movement of the weapon and helps keep the target in sight.

I'm not a physicist. I do know my ARs with low mass operating systems "shoot softer" than the ones with standard or heavy weight parts. It's another one of those ARFCOM "get both" things

Link Posted: 12/20/2014 4:08:20 PM EST
[Last Edit: 12/20/2014 4:10:03 PM EST by HeavyMetal]
Link Posted: 12/20/2014 5:12:03 PM EST
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Originally Posted By HeavyMetal:



Light BCG's are gamer parts. They compromise the reliability of the system and make it less tolerant of ammo variations and dirt. You need an adjustable gas system to get the lower recoil impulse. If you are not competing, it is not a good choice.
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Originally Posted By HeavyMetal:
Originally Posted By dshield55:
I'm hoping an Engineer or somebody else with a good grasp of physics can chime in on this.

I bought a Low Mass BCG because I wanted an AR with the least amount of recoil possible. I heard that heavy BCGs are better because they move slower making less recoil, but I also heard because there's more weight then there's more recoil because of more mass moving. I ended up buying the latter point of view, but now I'm thinking I'm an idiot.

What I can remember from Physics is that it shouldn't matter one way or another if it's a light or a heavy bcg. The amount of energy pushing on the BCG is the same, so so a lighter BCG just moves faster and a heavier BCG just moves slower and either way the momentum of the light or heavy BCG are virtually equally meaning the recoil hasn't changed one bit.

As part of my recoil strategy, I got an extra stength buffer spring, hydraulic buffer, and a tuneable gas block. The idea is that with the extra strength of the spring and tuning the gas just right, I can get it so that the buffer never really hits the back of receiver extension, and if it does, the hydraulic buffer will dampen that last bit of movement. I was aiming for the constant recoil principal of the AA12 shotgun and Ultimax 100 machine gun where the BCG never hits the rear and is fully arrested by the springs. In automatic fire, a slower rate of fire reduces recoil beyond the constant recoil principal because you can average the energy of all those shots in rapid recession. But in a semi-automatic where you're only shooting one at a time, that wouldn't make a difference.

So, the question is, could I have just as well went with a heavy BCG? Wouldn't the stronger springs and tuned down gas react the same to a heavier/slower bcg the way they would with a lighter/faster bcg? In your answer, let me know if you're an engineer or similar that has a background understanding the math and formulas behind all this stuff.



Light BCG's are gamer parts. They compromise the reliability of the system and make it less tolerant of ammo variations and dirt. You need an adjustable gas system to get the lower recoil impulse. If you are not competing, it is not a good choice.


This. Seems to me that lightweight action components have become a mainstream fad...and IMO it's just the wrong thing to do for a lot of AR15 owners.
Link Posted: 12/20/2014 5:18:08 PM EST
Here is an excerpt from the interview Dan Shea did with Jim Sullivan for Small Arms Review.

Jim Sullivan: Everyone thinks they know the reason a gun kicks. A bullet goes this way...

SAR: ...and Mr. Newton tells us that there will be an equal and opposite reaction....

Jim Sullivan: ...and it's true you can't violate that principle, Dan, but recoil is a measure of force times time. Let's take a simple bolt action rifle: The force of recoil is the same force that's driving the bullet, and it's for the same amount of time that it takes to drive that bullet. In other words, whatever amount of time it takes to accelerate that bullet from the back of the barrel to the front, and out through the front of the barrel, that amount of time is what that same force is pushing rearward against the gun and against the guy's shoulder. Okay, of course it kicks, but here's the thing: that time is very short, so the force is very high. Recoil is a function of force times time. In a machine gun, let's say it's firing 600 shots a minute, that's ten shots a second or one shot every one-tenth of a second. You have one-tenth of a second to deliver that force. If you're delivering it in the one-thousandth of a second that a bolt action rifle does, you're screwed. But if you can find a way to stretch it out, instead of a thousandth of a second, stretch that time out to a tenth of a second, that's 100 times longer. That means the force is one one-hundredth, and it's the force that moves you, not the time. It not only means that you can reduce that force to one one-hundredth; that's a hell of a reduction in force. It also means it becomes a constant force, because you time it so that it fires this shot and stretches all the recoil out until exactly the time it fires the next shot. So the force is not only a very small force, it is now constant. It doesn't hit you as a bunch of sharp impacts.
Link Posted: 12/20/2014 9:01:16 PM EST
Link Posted: 12/20/2014 9:20:11 PM EST
Different carrier weights (and adjusting the amount of gas) doesn't change the recoil at all. It CAN change how it feels, though. A lot of people say that a 9mm AR "recoils harder" than one in .223, but the physics says otherwise. What they're feeling is a VERY heavy bolt that spreads out the recoil impulse so you feel it for a longer period of time.

With a .223/5.56mm AR, if it has a dead-stock (and in-spec) carbine gas system and standard carbine spring and buffer, you will have "more than enough" gas, and a sufficiently heavy buffer to run the gun reliably with standard-power ammunition. A mid-length gas system changes the timing and a couple of other factors, but most importantly it delays the gas impulse until the gas pressure is lower, so it's less abrupt. You can feel the difference if you compare otherwise identical 16" carbines with mid- and carbine length system side by side.

Our 3-gun friend above pointed out how "they" do it - with light loads and as light a buffer and spring as they can get away with. That doesn't mean those ARs are going to be reliable with full power ammunition, In fact, they'll probably be real beasts with that kind of ammo.

By only changing to a low-mass BCG and heavy buffer and spring, you just changed which parts are doing what. You probably didn't feel a big difference between the stock parts and the new parts. Now if you'd kept a standard BCG and used a heavy buffer, you would see a difference. The heavy buffer would eat up some of the extra energy the gas system provides, and that usually "feels" lighter. In exchange, you might have issues with lighter loads not cycling or locking the bolt back on an empty magazine.
Link Posted: 12/20/2014 10:31:43 PM EST
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Originally Posted By GHPorter:
Different carrier weights (and adjusting the amount of gas) doesn't change the recoil at all. It CAN change how it feels, though. A lot of people say that a 9mm AR "recoils harder" than one in .223, but the physics says otherwise. What they're feeling is a VERY heavy bolt that spreads out the recoil impulse so you feel it for a longer period of time.

With a .223/5.56mm AR, if it has a dead-stock (and in-spec) carbine gas system and standard carbine spring and buffer, you will have "more than enough" gas, and a sufficiently heavy buffer to run the gun reliably with standard-power ammunition. A mid-length gas system changes the timing and a couple of other factors, but most importantly it delays the gas impulse until the gas pressure is lower, so it's less abrupt. You can feel the difference if you compare otherwise identical 16" carbines with mid- and carbine length system side by side.

Our 3-gun friend above pointed out how "they" do it - with light loads and as light a buffer and spring as they can get away with. That doesn't mean those ARs are going to be reliable with full power ammunition, In fact, they'll probably be real beasts with that kind of ammo.

By only changing to a low-mass BCG and heavy buffer and spring, you just changed which parts are doing what. You probably didn't feel a big difference between the stock parts and the new parts. Now if you'd kept a standard BCG and used a heavy buffer, you would see a difference. The heavy buffer would eat up some of the extra energy the gas system provides, and that usually "feels" lighter. In exchange, you might have issues with lighter loads not cycling or locking the bolt back on an empty magazine.
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Yes, he is correct. If you switch to the 3 gun recoil reduction method and dial it to your lightest possible load then run 5.56 loads not only will felt recoil increase dramatically but you will also have receiver damage and possibly damage to the buffer tube.

If this is for a home defense rifle, I'd leave it be since OP said it ejected at 3 o'clock. Range plinker I'd ask a shooting buddy if I could borrow a heavier buffer.
Link Posted: 12/20/2014 10:31:53 PM EST
Don't have to be a physicist to understand. Go throw a base ball, then throw a shot put. Which one is going to exhibit more force when you start, stop, change direction, then stop again ( one cycle of the bcg)? While the heavier bcg will travel slower it retains more energy due to its mass. While you can get a low mass carrier to slap back and fourth as hard as a full mass carrier, it will be greatly reduced if you use an adjustable gas block. That way you can tune down the gas pressure applied to the bolt. Less energy applied to the BCG = less energy transferred to the shooter.

Also this applies to the movement associated with operating the action. It has nothing to do with the recoil caused by the bullet exiting the barrel. You use a muzzle brake to reduce that.
Link Posted: 12/21/2014 10:09:45 AM EST
For every action there is an equal and opposite reaction.:Winchester Defender 223 Rem 77 grain* 2500 ft per sec Ballistic Coefficient 0.319.
You are looking at this and have made it too complicated. The discharged cartridge produces a recoil that has a measurable force. That energy is designed to be used to slide back the bolt, eject the empty shell, and continue to slide back to the shock chamber which has the springs in place to return the bolt and push up and chamber the next fresh round. The better Balanced the forces are the smother the operation. However at what cost? The parts that are initially used for the smooth operation will wear and "get tired" and then instead of ejecting the spent round the result will be a stove pipe. For every action there is an equal and opposite action. Simple to figure, then add for wear. If you add 15% to your resistance the parts will serve you well for at least a thousand rounds but look closely at the weakest link, the return spring, and be prepared to replace it in the future.
Link Posted: 12/21/2014 5:59:18 PM EST
The heavier bolt system has less recoil. I was dead wrong, and apparently most the 3gunners are dead wrong. Jim Sullivan has it right. I wasn't happy with the answers and rationale I was getting here, so I reposted this question to a Physics forum. They all agree the heavier system has less recoil. The scientific explanation is below, but first some bloviating.

It's confusing because if you go to JP's website and read about their low mass buffers and bolt carriers they say they reduce recoil. If you go to VLTOR's website and read about their A5 system that includes heavier parts, they say that it reduces recoil. I initially believed the 3gunners who mostly say the lighter system reduces recoil because when both schools of thought are saying there's is the way to reduce the recoil best, I went with the 3gunners opinion. And I payed $200 for a low mass carrier that I'm now going to have to sell.
Link Posted: 12/21/2014 5:59:58 PM EST
Here is the scientific reasoning. The formula is dP/dt=F. That's a calculus based derivative formula that's known as the Law of Conservation of Momentum. It's not algebra, so don't bother finding a value of d and trying to plug it and solf for F. P is momentum, t is time, and F is force. This formula says Force is the derivative of momentum in relation to time. Momentum is Mass time Velocity. And here's how it plays out in simple math that we can all understand.

You have your gas system tuned, and you have your spring installed. At that point, as long as you don't change the gas settings or change the springs, the bolt system is going to travel the exact same distance down the spring, and still have the exact same amount of force when it hits the rear of the receiver extension. That remains constant. However, lets assume for simplicity of math that a light bolt system (BCG and Buffer) weighs 8oz and a heavy bolt system weighs 12oz. If the light system 8oz system cycles at 900rpm, then the 12 oz WILL (not if, WILL) cycle at 600 rpm. It follows then that the 8oz system flying at 900rpms has 1.5xtimes the recoil impulse of the 12oz system flying at 600rpm.
Link Posted: 12/21/2014 6:00:56 PM EST
In semi-auto fire, this all happens really quickly. At 900rpms, the recoil is felt for 0.033 seconds (1/30th of a second.) At 600rpms, the recoil is felt for 0.05 seconds (1/20th of a second.) Those are times that we couldn't really differentiate between at all. There is no way you would know that the heavy gun was recoiling against you for 1/20th of a second but the light gun was recoiling against you for 1/30th of a second. But, you should be able to feel that the lighter gun was recoiling against you 1.5 times more than the heavier gun. Don't get me wrong, the total amount of force over time transfered to your shoulder is exactly the same (that's the function of your gas settings,) but all you really feel is the peak force which could be as much as 50% more with the lighter carrier (conversely it would be 33% less with the heavier.) Friction wasn't factored it, it was deemed negligible. The heavier bolt carriers typically have more surface area causing more friction which should slow it down just a bit more than our simple formula would say. That's why you might have to retune your gas if you change weights of the system.

So, if you're a 3 gunner and reading this and feeling dumb (you should feel dumb, because I feel dumb that I bought a JP low mass carrier) you probably just felt like it kicked less because your loads generally don't have much of a kick anyway and it was/is reviewers bias. The science is just not on your side. Here is the video of Jim Sullivan calling you crazy for thinking the low masses would recoil less. I watched this a couple weeks ago and this is what caused me to rethink everything. Jim is an engineer. He worked with Eugene Stoner in developing the AR15 and went on to develop the Ultimax 100 light machine gun for the Singaporan Army. That gun is famous for it's lack of recoil.

https://www.full30.com/video/9b50f8a825ab510b4c227c7b32a76bc1
Link Posted: 12/21/2014 6:17:10 PM EST
[Last Edit: 12/21/2014 6:27:19 PM EST by jbooker82]
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Originally Posted By dshield55:
The heavier bolt carriers typically have more surface area causing more friction which should slow it down just a bit more than our simple formula would say. That's why you might have to retune your gas if you change weights of the system.
View Quote


Or because less weight requires less energy to travel the same distance at the same speed. Less weight and less energy = less opposite and equal reaction. (I.E. Recoil).

Force = Mass x Acceleration. Your not increasing the acceleration of the bolt because your dialing down the gas system, so a lighter bolt at the same speed = less force.
Link Posted: 12/21/2014 6:19:08 PM EST
Here is what I do to reduce felt recoil in a carbine length gas system AR, I use a M16 BCG, and a H2 buffer. That is it. It is simple and it works well for me. In my mid-length gas system ARs, I run a M16 BCG and a standard buffer. Very simple and it works very well. To reduce recoil even more, there are a few buffer springs that will do the job as well. My ARs are not built for just punching paper. I shoot full-powered 5.56 NATO ammo in them and they all run reliably.
Link Posted: 12/21/2014 6:44:03 PM EST
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Originally Posted By jbooker82:


Or because less weight requires less energy to travel the same distance at the same speed. Less weight and less energy = less opposite and equal reaction. (I.E. Recoil).

Force = Mass x Acceleration. Your not increasing the acceleration of the bolt because your dialing down the gas system, so a lighter bolt at the same speed = less force.
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Originally Posted By jbooker82:
Originally Posted By dshield55:
The heavier bolt carriers typically have more surface area causing more friction which should slow it down just a bit more than our simple formula would say. That's why you might have to retune your gas if you change weights of the system.


Or because less weight requires less energy to travel the same distance at the same speed. Less weight and less energy = less opposite and equal reaction. (I.E. Recoil).

Force = Mass x Acceleration. Your not increasing the acceleration of the bolt because your dialing down the gas system, so a lighter bolt at the same speed = less force.


You're right that less weight requires less energy to travel the same distance, what you're missing is that the same amount of energy is dumped on the system whether it's light weight or heavy weight. If, and only if, you change the gas settings will it change how much energy is being dumped on the system. So, all things being equal the lighter bolt carrier will travel faster and it's going to impart more recoil during that shorter time of travel.
Link Posted: 12/21/2014 6:46:22 PM EST
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Originally Posted By jbooker82:


Or because less weight requires less energy to travel the same distance at the same speed. Less weight and less energy = less opposite and equal reaction. (I.E. Recoil).

Force = Mass x Acceleration. Your not increasing the acceleration of the bolt because your dialing down the gas system, so a lighter bolt at the same speed = less force.
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Originally Posted By jbooker82:
Originally Posted By dshield55:
The heavier bolt carriers typically have more surface area causing more friction which should slow it down just a bit more than our simple formula would say. That's why you might have to retune your gas if you change weights of the system.


Or because less weight requires less energy to travel the same distance at the same speed. Less weight and less energy = less opposite and equal reaction. (I.E. Recoil).

Force = Mass x Acceleration. Your not increasing the acceleration of the bolt because your dialing down the gas system, so a lighter bolt at the same speed = less force.


You're also missing that if you tune down the gas system to slow the light weight bolt down, it's not going to have enough energy to compress the spring back which means it wouldn't cycle.
Link Posted: 12/21/2014 6:47:35 PM EST
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Originally Posted By jbooker82:


Or because less weight requires less energy to travel the same distance at the same speed. Less weight and less energy = less opposite and equal reaction. (I.E. Recoil).

Force = Mass x Acceleration. Your not increasing the acceleration of the bolt because your dialing down the gas system, so a lighter bolt at the same speed = less force.
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Originally Posted By jbooker82:
Originally Posted By dshield55:
The heavier bolt carriers typically have more surface area causing more friction which should slow it down just a bit more than our simple formula would say. That's why you might have to retune your gas if you change weights of the system.


Or because less weight requires less energy to travel the same distance at the same speed. Less weight and less energy = less opposite and equal reaction. (I.E. Recoil).

Force = Mass x Acceleration. Your not increasing the acceleration of the bolt because your dialing down the gas system, so a lighter bolt at the same speed = less force.


You're also missing that if you tune down the gas system to slow the light weight bolt down, it's not going to have enough energy to compress the spring back which means it wouldn't cycle. So although you're right that slowing down the lighter system would have less recoil, that's not a functional option.
Link Posted: 12/21/2014 6:50:24 PM EST
[Last Edit: 12/21/2014 6:53:56 PM EST by bionic589]
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Originally Posted By dshield55:


You're also missing that if you tune down the gas system to slow the light weight bolt down, it's not going to have enough energy to compress the spring back which means it wouldn't cycle.
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Originally Posted By dshield55:
Originally Posted By jbooker82:
Originally Posted By dshield55:
The heavier bolt carriers typically have more surface area causing more friction which should slow it down just a bit more than our simple formula would say. That's why you might have to retune your gas if you change weights of the system.


Or because less weight requires less energy to travel the same distance at the same speed. Less weight and less energy = less opposite and equal reaction. (I.E. Recoil).

Force = Mass x Acceleration. Your not increasing the acceleration of the bolt because your dialing down the gas system, so a lighter bolt at the same speed = less force.


You're also missing that if you tune down the gas system to slow the light weight bolt down, it's not going to have enough energy to compress the spring back which means it wouldn't cycle.


So OP now that you know more than everyone else regarding recoil impulse, will you be correcting all those dumb three gunners?

ETA: I found this, but I have no idea what it could ever be used for.

Link Posted: 12/21/2014 6:56:14 PM EST
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Originally Posted By dshield55:


You're also missing that if you tune down the gas system to slow the light weight bolt down, it's not going to have enough energy to compress the spring back which means it wouldn't cycle. So although you're right that slowing down the lighter system would have less recoil, that's not a functional option.
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Originally Posted By dshield55:
Originally Posted By jbooker82:
Originally Posted By dshield55:
The heavier bolt carriers typically have more surface area causing more friction which should slow it down just a bit more than our simple formula would say. That's why you might have to retune your gas if you change weights of the system.


Or because less weight requires less energy to travel the same distance at the same speed. Less weight and less energy = less opposite and equal reaction. (I.E. Recoil).

Force = Mass x Acceleration. Your not increasing the acceleration of the bolt because your dialing down the gas system, so a lighter bolt at the same speed = less force.


You're also missing that if you tune down the gas system to slow the light weight bolt down, it's not going to have enough energy to compress the spring back which means it wouldn't cycle. So although you're right that slowing down the lighter system would have less recoil, that's not a functional option.


Yes it is an option. There are many people on here who run a light weight bolt carrier group with an adjustable gas block to control gas pressure / bcg speed.
Link Posted: 12/21/2014 7:00:35 PM EST
[Last Edit: 12/21/2014 7:01:20 PM EST by jbooker82]
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Originally Posted By bionic589:


So OP now that you know more than everyone else regarding recoil impulse, will you be correcting all those dumb three gunners?

ETA: I found this, but I have no idea what it could ever be used for.

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Originally Posted By bionic589:
Originally Posted By dshield55:
Originally Posted By jbooker82:
Originally Posted By dshield55:
The heavier bolt carriers typically have more surface area causing more friction which should slow it down just a bit more than our simple formula would say. That's why you might have to retune your gas if you change weights of the system.


Or because less weight requires less energy to travel the same distance at the same speed. Less weight and less energy = less opposite and equal reaction. (I.E. Recoil).

Force = Mass x Acceleration. Your not increasing the acceleration of the bolt because your dialing down the gas system, so a lighter bolt at the same speed = less force.


You're also missing that if you tune down the gas system to slow the light weight bolt down, it's not going to have enough energy to compress the spring back which means it wouldn't cycle.


So OP now that you know more than everyone else regarding recoil impulse, will you be correcting all those dumb three gunners?

ETA: I found this, but I have no idea what it could ever be used for.



Or this, you only have 5 different springs to chose from, and you can you can mix and match weight material.
http://www.jprifles.com/buy.php?item=JPSCS-15K


Aslo who would have though that the 3 gunners had it all wrong all these years. Instead of light weight Ti systems they should have been running Tungsten BCG all a long.
Link Posted: 12/21/2014 7:16:52 PM EST
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Originally Posted By bionic589:

So OP now that you know more than everyone else regarding recoil impulse, will you be correcting all those dumb three gunners?

ETA: I found this, but I have no idea what it could ever be used for.

View Quote


Light weight systems still have their place for 3 gun because it saves a few ounces. If I did a JP Low mass plus their lightweigh buffer I think the whole thing is just over 8oz. On the other hand if I go with a full auto bolt carrier and 6.8 oz VLTOR A5 buffer system, I'd be up over a pound. So really I just added half a pound to reduce the recoil of the bolt system in half. And that's just that one aspect of recoil. Has nothing to do with the bullet and gas leaving the barrel at the front. So I guess it's just a matter of preference when I do agree that these systems have not a lot of recoil to begin with. But, it's for sure settled in my mind that if somebody says their light weight bolt setup has less recoil that they're making it up due to reviewers bias.


I didn't consider a lighter weight than normal spring because they increase the speed and probability of hitting the rear of the receiver. However, if I tuned down the gas that wouldn't be the case and then less energy is being dumped into the system and less recoil. Interesting though worth some experimenting with.
Link Posted: 12/21/2014 7:18:26 PM EST
My specific application is a 6.5 Grendel. I'm going to be shooting pigs near a feeder from a blind. My goal is to shoot as many as possible before they take off. I wanted a system where it has the least recoil possible so it doesn't knock me off my sights. I'll be resting the gun on a ledge so for me it doesn't matter if it's heavier. I was at 9lbs with the low mass system and now I'm going to be at 9.5lbs. No biggie for me.
Link Posted: 12/21/2014 7:23:34 PM EST
Who knew that Jerry Miculek didn't have a clue how to set up his AR.

Damn, I wish I could makeover a Million dollars a year shooting and not understand how to set up my weapon
Link Posted: 12/21/2014 7:24:31 PM EST
Since you have the Low Mass JP Bolt Carrier try it with a stock spring and buffer. Load you magazine with 1 round and fire. If it locks the bolt back then turn it down a little more. Keep doing this until it fails to lock the bolt back on an empty mag. Then increase the gas until it locks back on the empty mag.

Then do the same for a full mass or a weighted carrier.
Link Posted: 12/21/2014 8:47:09 PM EST
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Originally Posted By PursuitSS:
Who knew that Jerry Miculek didn't have a clue how to set up his AR.

Damn, I wish I could makeover a Million dollars a year shooting and not understand how to set up my weapon
View Quote


I love Miculek. I would tell him he's dead wrong on this right to his face though. The low mass thing has apparently been spread by confusion. If Jim Sullivan who actually made the gun and is an engineer thinks it's less recoil with the higher weights, why would his opinion be less than Miculek who just shoots it? But the actual science is on the side of Sullivan, not Miculek.

I got caught up in the low mass thing too. I have a slick side Mega upper that i bought and I had the choice of getting the one with or without a side charging slot, and I chose the one without the side charging slot because I was sure I was going with the JP Low Mass bolt which wouldnt be robust enough to machine the handle into. Now I'm probably going to have to get to a heavier side charging bolt from Young Manufacturing instead to get the results I thought I was going to get from JP and I have to mill the slot myself. I wasn't the only one fooled by the low mass fad.
Link Posted: 12/21/2014 9:34:33 PM EST
Let us know how your heavy weight BCG works out.

http://youtu.be/-0LEDFKQywA
Link Posted: 12/21/2014 9:44:08 PM EST
OP the science you keep quoting is exactly what makes a TUNED low mass system recoil less. It's been posted a few times in this thread already but I will repeat:

A lower reciprocating mass with a TUNED gas system (small port, adjustable gas block, etc.) has less recoil because there is less force, e.g. less gas pushing less weight.

3 gunners don't use ONLY a low mass bcg to reduce recoil. It's used in conjunction with reduced gas systems and reduced power loads.

All things being equal, a low mass bcg will have a shorter recoil impulse with a greater magnitude of force because the force is imparted in a shorter duration.

However, it's foolish to use a low mass bcg without adjusting the gas, ammo, and springs.
Link Posted: 12/21/2014 9:47:14 PM EST
If you REALLY want to know which has the least recoil you need to go to extremes......

Standard Colt Carrier





Smith Enterprises Aluminum

Link Posted: 12/21/2014 10:22:51 PM EST
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Originally Posted By azoutdoorsman:
OP the science you keep quoting is exactly what makes a TUNED low mass system recoil less. It's been posted a few times in this thread already but I will repeat:

A lower reciprocating mass with a TUNED gas system (small port, adjustable gas block, etc.) has less recoil because there is less force, e.g. less gas pushing less weight.

3 gunners don't use ONLY a low mass bcg to reduce recoil. It's used in conjunction with reduced gas systems and reduced power loads.

All things being equal, a low mass bcg will have a shorter recoil impulse with a greater magnitude of force because the force is imparted in a shorter duration.

However, it's foolish to use a low mass bcg without adjusting the gas, ammo, and springs.
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You're right that a TUNED system will recoil less. Where you err is the low mass part of it. Conservation of Momentum (momentum being Mass x Velocity) means that with a decrease in mass, your velocity increases. Either way, the gas is the only thing that acted on the bolt/buffer system and will create the same amount of Momentum for the heavy or the low mass system and end up compressing the spring the same distance. Because less gas is used in a tuned system, it will have less recoil for both heavy and light masses, but the heavier mass will still exhibit less recoil than the light system with the same amount of gas.
Link Posted: 12/21/2014 10:25:54 PM EST
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Originally Posted By PursuitSS:
If you REALLY want to know which has the least recoil you need to go to extremes......

Standard Colt Carrier

Smith Enterprises Aluminum

View Quote


Nice!!! I think I'm gonna try to get my hands on one of those. I'm going to test this some time in the next couple weeks to quantify the recoil of the low mass versus heavy mass systems.
Link Posted: 12/21/2014 10:56:34 PM EST
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Originally Posted By dshield55:


You're right that a TUNED system will recoil less. Where you err is the low mass part of it. Conservation of Momentum (momentum being Mass x Velocity) means that with a decrease in mass, your velocity increases. Either way, the gas is the only thing that acted on the bolt/buffer system and will create the same amount of Momentum for the heavy or the low mass system and end up compressing the spring the same distance. Because less gas is used in a tuned system, it will have less recoil for both heavy and light masses, but the heavier mass will still exhibit less recoil than the light system with the same amount of gas.
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Originally Posted By dshield55:
Originally Posted By azoutdoorsman:
OP the science you keep quoting is exactly what makes a TUNED low mass system recoil less. It's been posted a few times in this thread already but I will repeat:

A lower reciprocating mass with a TUNED gas system (small port, adjustable gas block, etc.) has less recoil because there is less force, e.g. less gas pushing less weight.

3 gunners don't use ONLY a low mass bcg to reduce recoil. It's used in conjunction with reduced gas systems and reduced power loads.

All things being equal, a low mass bcg will have a shorter recoil impulse with a greater magnitude of force because the force is imparted in a shorter duration.

However, it's foolish to use a low mass bcg without adjusting the gas, ammo, and springs.


You're right that a TUNED system will recoil less. Where you err is the low mass part of it. Conservation of Momentum (momentum being Mass x Velocity) means that with a decrease in mass, your velocity increases. Either way, the gas is the only thing that acted on the bolt/buffer system and will create the same amount of Momentum for the heavy or the low mass system and end up compressing the spring the same distance. Because less gas is used in a tuned system, it will have less recoil for both heavy and light masses, but the heavier mass will still exhibit less recoil than the light system with the same amount of gas.
I believe you are correct and so is the poster above you.
You need less gas to drive a lighter bcg. So, you can tune down the gas lower on a lighter BCG vs a heavier one. (assuming we are going for reliability here) And yes, given that you have an equal amount of gas, heavier buffer will feel a softer recoil impulse.

Sorry, i have been drinking, and tomorrow is going to be a long day at work, finishing my last engineering change notice before I go on vacation.
Link Posted: 12/21/2014 11:06:51 PM EST
Alright...my eyes crossed a couple of times while trying to keep up here, but clarify this for me. I take it that the OP is not a 3-gun competitor and has no desire to be one. On of the key elements in a 3-gunner's weapon is a reduced load in most cases...obviously along with his lighter BCG and matching buffer/spring setup. This allows the 3-gunner to maintain better, and I guess faster, fire control...with the added benefit of a slightly lighter weapon. The 3-gunner has achieved lower recoil by balancing these elements which work quite well for his arena. Now, the heavier BCG with the properly matching buffer/spring system and hotter, more normal ammo has about the same recoil as the 3-gunner. However, with the 3-gunner you're shooting reduced loads which probably don't jive as the best tool for the job for the application the OP was stating...or that most of us might employ for hunting and longer range applications.

Now here's where I need the clarification. If the OP installs the lighter BCG and even a suitable buffer/spring but doesn't reduce the ammo load, he's going to get more recoil...right? His quoted science on that issue appears sound. Not sure I understand why the OP thinks the 3-gunners have it all wrong in their claim that they reduce recoil with their method. It reduces recoil at the expense of ballistic efficiency on the target. But...that's not what they're after, and I didn't see where that claim was made. At the moment, my takeaway from this is that you don't necessarily want a light BCG and buffer combo with full power loads in a hunting, combat, or similar scenario. However, we are talking about a .223/5.56 cartridge here, and being a .308 AR guy, I have not idea how much additional recoil will be experienced with the .223/5.56.
Link Posted: 12/21/2014 11:18:30 PM EST
[Last Edit: 12/22/2014 12:12:02 AM EST by jbooker82]
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Originally Posted By dshield55:

You're right that a TUNED system will recoil less. Where you err is the low mass part of it. Conservation of Momentum (momentum being Mass x Velocity) means that with a decrease in mass, your velocity increases. Either way, the gas is the only thing that acted on the bolt/buffer system and will create the same amount of Momentum for the heavy or the low mass system and end up compressing the spring the same distance. Because less gas is used in a tuned system, it will have less recoil for both heavy and light masses, but the heavier mass will still exhibit less recoil than the light system with the same amount of gas.
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The part your failing to realize is that the low mass bcg and the full mass bcgs require different amounts of gas to operate. The heavier full mass carrier requires more gas to operate because it is a heavier object to start moving, and uses stiffer recoil springs, and a heavier buffer. Once its moving it also requires more energy to stop (heavier buffer, heavier recoil spring, and a harder slam when the bolt engages the barrel extension)

The gas is the energy that operates the system. Less gas is less energy. Elementary Science class teaches The Conservation of Energy: Energy is neither created nor destroyed. So how does the low mass system create more energy, when it has less energy input to operate the system?
Link Posted: 12/21/2014 11:32:08 PM EST
[Last Edit: 12/21/2014 11:36:41 PM EST by jbooker82]
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Originally Posted By ARTNC10:.

Now here's where I need the clarification. If the OP installs the lighter BCG and even a suitable buffer/spring but doesn't reduce the ammo load, he's going to get more recoil...right? His quoted science on that issue appears sound. Not sure I understand why the OP thinks the 3-gunners have it all wrong in their claim that they reduce recoil with their method. It reduces recoil at the expense of ballistic efficiency on the target. But...that's not what they're after, and I didn't see where that claim was made. At the moment, my takeaway from this is that you don't necessarily want a light BCG and buffer combo with full power loads in a hunting, combat, or similar scenario. However, we are talking about a .223/5.56 cartridge here, and being a .308 AR guy, I have not idea how much additional recoil will be experienced with the .223/5.56.
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They operate with less gas pressure and volume by using an adjustable gas block. It limits the amount of energy applied to the bcg, so your not excessily slamming it around.

The lighter / weaker loads would reduce the recoil caused by the bullet and gasses leaving the muzzle. Yes a weaker load would also equal a lower gas system pressure for the action, but you can dial down the gas pressure with an adjustabe gas block on regular full power loads.

There is 3 major recoil forces when firing a semi or full auto firearm. The launch or starting motion of the bullet (ignition), the cycling of the action, and the gasses leaving the muzzle.
Link Posted: 12/22/2014 12:10:43 AM EST
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Originally Posted By jbooker82:

The part your failing to realize is that the low mass bcg and the full mass bcgs require different amounts of gas to operate.
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That's actually what I'm disputing. They DO NOT require different amount of gases. If a certain powder load and gas setting will send one mass of bcg/buffers to the rear of the receiver extension, it will send ANY mass to the rear of the receiver extension. All thats being traded is mass for velocity.

This isn't a cannon type problem where you need more charge to throw the a heavier mass the same distance as a lighter mass. What is different about this problem is that we're attempting to compress a spring. The spring can care less if it's a lighter/faster mass versus a heavier/slower mass, the same strength spring is going to be compressed the exact same distance for the same amount of momentum. It's the gas setting that determines the momentum, not the weight of the bcg, so the weights of the bcg are interchangeable for the same gas settings and spring.
Link Posted: 12/22/2014 12:15:20 AM EST
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Originally Posted By ARTNC10:

Now here's where I need the clarification. If the OP installs the lighter BCG and even a suitable buffer/spring but doesn't reduce the ammo load, he's going to get more recoil...right? His quoted science on that issue appears sound. Not sure I understand why the OP thinks the 3-gunners have it all wrong in their claim that they reduce recoil with their method.
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No, I agree that a reduced powder charge creates less recoil.
Link Posted: 12/22/2014 12:51:28 AM EST
[Last Edit: 12/22/2014 12:57:38 AM EST by jbooker82]
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Originally Posted By dshield55:


That's actually what I'm disputing. They DO NOT require different amount of gases. If a certain powder load and gas setting will send one mass of bcg/buffers to the rear of the receiver extension, it will send ANY mass to the rear of the receiver extension. All thats being traded is mass for velocity.

This isn't a cannon type problem where you need more charge to throw the a heavier mass the same distance as a lighter mass. What is different about this problem is that we're attempting to compress a spring. The spring can care less if it's a lighter/faster mass versus a heavier/slower mass, the same strength spring is going to be compressed the exact same distance for the same amount of momentum. It's the gas setting that determines the momentum, not the weight of the bcg, so the weights of the bcg are interchangeable for the same gas settings and spring.
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Originally Posted By dshield55:
Originally Posted By jbooker82:

The part your failing to realize is that the low mass bcg and the full mass bcgs require different amounts of gas to operate.


That's actually what I'm disputing. They DO NOT require different amount of gases. If a certain powder load and gas setting will send one mass of bcg/buffers to the rear of the receiver extension, it will send ANY mass to the rear of the receiver extension. All thats being traded is mass for velocity.

This isn't a cannon type problem where you need more charge to throw the a heavier mass the same distance as a lighter mass. What is different about this problem is that we're attempting to compress a spring. The spring can care less if it's a lighter/faster mass versus a heavier/slower mass, the same strength spring is going to be compressed the exact same distance for the same amount of momentum. It's the gas setting that determines the momentum, not the weight of the bcg, so the weights of the bcg are interchangeable for the same gas settings and spring.


Yes they do require different levels of gas to operate. Its not about needing more for the heavy setup. Its about getting by with less running the light weight setup. You can run a light carrier with full gas and it will slap the carrier around excessively. If you try to run a full mass setup at the gas level that the low mass carrier barley functions, the full mass setup wont function properly.

The spring and buffer that you run in a low mass setup is lighter than what you normally run with a full mass carrier.

Link Posted: 12/22/2014 12:57:06 AM EST
Link Posted: 12/22/2014 1:08:03 AM EST
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Originally Posted By RustedAce:

This is incorrect.
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Originally Posted By RustedAce:
Originally Posted By dshield55:They DO NOT require different amount of gases. If a certain powder load and gas setting will send one mass of bcg/buffers to the rear of the receiver extension

This is incorrect.


It's absolutely correct. The spring is fighting the momentum. It's not fighting the mass, it's not fighting the velocity. It's fighting the total momentum. The same gas settings that send an 8 oz system at 900rpms will send a 12oz system at 600rpms. The momentum created is going to be the exact same given the same gas settings.

if the spring compresses 4.0 inches with the 8oz system, it will compress 4inches with the 12oz system. Not 3.9 inches or 3.8 inches or 3.7 inches, it will compress 4.0 inches. The only thing that might change is the friction differences between a heavier bolt and a lighter bolt because the heavier ones tend to have more surface area. With proper lubrication and being that they only travel a small distance, it should be negligible.

Link Posted: 12/22/2014 1:10:52 AM EST
Link Posted: 12/22/2014 1:14:16 AM EST
[Last Edit: 12/22/2014 1:15:02 AM EST by jbooker82]
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Originally Posted By RustedAce:

Just go put heavier buffers in a gun till it stops running, it will.by
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Originally Posted By RustedAce:
Originally Posted By dshield55:

It's absolutely correct. The spring is fighting the momentum. It's not fighting the mass, it's not fighting the velocity. It's fighting the total momentum. The same gas settings that send an 8 oz system at 900rpms will send a 12oz system at 600rpms. The momentum created is going to be the exact same given the same gas settings.

if the spring compresses 4.0 inches with the 8oz system, it will compress 4inches with the 12oz system. Not 3.9 inches or 3.8 inches or 3.7 inches, it will compress 4.0 inches. The only thing that might change is the friction differences between a heavier bolt and a lighter bolt because the heavier ones tend to have more surface area. With proper lubrication and being that they only travel a small distance, it should be negligible.


Just go put heavier buffers in a gun till it stops running, it will.by

Yea since weight apparently doesnt matter.
Link Posted: 12/22/2014 1:27:30 AM EST
[Last Edit: 12/22/2014 1:28:02 AM EST by Regalkismet]
Someone SHIELD me from watching someone go full retard.
Link Posted: 12/22/2014 1:30:52 AM EST
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Originally Posted By dshield55:
The spring is fighting the momentum. It's not fighting the mass, it's not fighting the velocity.
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Hahahaha. This is great since Momentum = Mass × Velocity.
Link Posted: 12/22/2014 1:34:46 AM EST
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Originally Posted By jbooker82:

Hahahaha. This is great since Momentum = Mass × Velocity.
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Originally Posted By jbooker82:
Originally Posted By dshield55:
The spring is fighting the momentum. It's not fighting the mass, it's not fighting the velocity.

Hahahaha. This is great since Momentum = Mass × Velocity.

Science.....how does that stuff work again.
Link Posted: 12/22/2014 1:57:43 AM EST
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Originally Posted By RustedAce:
[Just go put heavier buffers in a gun till it stops running, it will.
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You obviously can't take the weight to infinity and you can't take the weight to zero. But swapping total weights of BCG and Buffer between 8oz and 16oz there shouldn't be any reliability issues for the same gas settings. I'd consider that you may have to tune up gas just a hair to overcome friction for increased surface area.

I'll be testing it out later in the week either way and measuring the recoil with sensors. Right now the lightest I can go with parts on hand is 8 oz and the heaviest I can go is 14oz. I'll test more extremes later.
Link Posted: 12/22/2014 3:59:03 AM EST
[Last Edit: 12/22/2014 4:29:59 AM EST by dshield55]
Here's the kinetic energy equations that relate to springs. E=(1/2)mv^2 and (1/2)mv^2=(1/2)kx^2.

^2 means squared, m = mass, v=volume, k= spring constant, and x=distance.

As you see from there the (mass x volume) is what determines how far the spring travels. Since (mass x volume) is MOMENTUM, it's the Momentum that determines the travel distance down the spring.

The force of the gas on the carrier is a momentary burst that is the exact same for a light carrier and a heavy carrier. I'd have to get into the calculus integral measuring the force over that small period to prove it, but then I'd have to teach calculus here first so everyone can follow along, so just trust me that it's equal.

That being said, the same force that created the momentum of the 8oz carrier going 900rpms would push the 12oz carrier at 600rpms. And sence the momentums are the same, the spring will be compressed the exact same distance. If you dialed the gas down and decreased the spring strength to get the 8oz carrier to 500rpms, then the 12oz carrier would be 375rpms and the 16 ounce system would be 250rpms.

If you have to increase the gas at all for a heavier bolt, it's only to overcome differences in friction. They're not going to be big differences though so you'd have to gas the heavier one just slightly.

But, if you did have to increase the gas slightly to run the heavier bolt, it's way worth it. You would reduce the recoil of the reciprocating mass 33% or 50% assuming a base line 8oz system versus a 12oz or 16oz system. How would that not be worth it?
Link Posted: 12/22/2014 4:31:10 AM EST
starting at about 6.20 of this video. James Sullivan addresses this topic.
Link Posted: 12/22/2014 6:20:22 AM EST
[Last Edit: 12/22/2014 6:30:49 AM EST by Willz]
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Originally Posted By HeyCoach:



Yes, he is correct. If you switch to the 3 gun recoil reduction method and dial it to your lightest possible load then run 5.56 loads not only will felt recoil increase dramatically but you will also have receiver damage and possibly damage to the buffer tube.

If this is for a home defense rifle, I'd leave it be since OP said it ejected at 3 o'clock. Range plinker I'd ask a shooting buddy if I could borrow a heavier buffer.
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Originally Posted By HeyCoach:
Originally Posted By GHPorter:
Different carrier weights (and adjusting the amount of gas) doesn't change the recoil at all. It CAN change how it feels, though. A lot of people say that a 9mm AR "recoils harder" than one in .223, but the physics says otherwise. What they're feeling is a VERY heavy bolt that spreads out the recoil impulse so you feel it for a longer period of time.

With a .223/5.56mm AR, if it has a dead-stock (and in-spec) carbine gas system and standard carbine spring and buffer, you will have "more than enough" gas, and a sufficiently heavy buffer to run the gun reliably with standard-power ammunition. A mid-length gas system changes the timing and a couple of other factors, but most importantly it delays the gas impulse until the gas pressure is lower, so it's less abrupt. You can feel the difference if you compare otherwise identical 16" carbines with mid- and carbine length system side by side.

Our 3-gun friend above pointed out how "they" do it - with light loads and as light a buffer and spring as they can get away with. That doesn't mean those ARs are going to be reliable with full power ammunition, In fact, they'll probably be real beasts with that kind of ammo.

By only changing to a low-mass BCG and heavy buffer and spring, you just changed which parts are doing what. You probably didn't feel a big difference between the stock parts and the new parts. Now if you'd kept a standard BCG and used a heavy buffer, you would see a difference. The heavy buffer would eat up some of the extra energy the gas system provides, and that usually "feels" lighter. In exchange, you might have issues with lighter loads not cycling or locking the bolt back on an empty magazine.



Yes, he is correct. If you switch to the 3 gun recoil reduction method and dial it to your lightest possible load then run 5.56 loads not only will felt recoil increase dramatically but you will also have receiver damage and possibly damage to the buffer tube.

If this is for a home defense rifle, I'd leave it be since OP said it ejected at 3 o'clock. Range plinker I'd ask a shooting buddy if I could borrow a heavier buffer.


That is where the adjustable gas block comes into play. If you shoot full power loads you dial down the gas so it's not battering your receiver and buffer set up.
You turn it down so the BCG will just lock back on the last round in a mag.
Ideal set up has the system balanced or tuned so the gas will just lock back the BCG on the last shot and just enough buffer spring force to return the BCG and chamber a round without slamming the BCG into battery. The full recoil impulse or felt recoil of the AR is not just the rearward system movement but that felt from the return to battery as well. Both can disturb your sight picture forcing you to re-aquire your sights, which takes time and competitors are fighting the clock as well as trying to be accurate.



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