========= INTRODUCTION ===========

There are plenty of manufacturers out there claiming that their product will help reduce recoil.  Some of those claims are complete bullshit, some of them are true, and some of them are true but only under certain other assumptions that they haven't told you.

In reading up on this subject, I have encountered many claims about how to reduce recoil.  For example:

* Dial down your adjustable gas block...it will slow down your BCG momentum, less recoil!
* Use a lighter BCG...it will reduce the momentum of moving parts, meaning less recoil!
* Use a heavier BCG...its momentum is subtracted from your gun's recoil, or it draws out the recoil over a longer period of time making it more manageable!
* Reduce the weight of your buffer, lighter moving parts means less recoil!
* If using a lighter BCG, then increase the weight of your buffer!
* If using a lighter BCG, then also use a lighter buffer!
* Increase the spring tension of your buffer
* Decrease spring tension of your buffer
* Use a compensator
* Increase the weight of your rifle's non moving parts
* A semi-auto gun recoils more than a bolt action precisely because the BCG absorbs some of the recoil.  Therefore, if we reduce the weight of the BCG, we are going to get closer to recoil of a bolt action.

Sounds like a bunch of contradictory information, doesn't it?  So how do we know who's right??

Well, the first thing we can do is look at what the pros do.  Here's what the professional 3-gunners do to reduce recoil:
1) Reduce weight of BCG
2) Reduce weight of buffer
3) Reduce spring tension
4) Use a compensator

...But are they really right?  If so, we should understand why from a physics perspective.  I recently came across some folks who tried to bring in some physics forum experts to analyze the problem, and they all agreed -- the 3-gunners were WRONG.  They concluded that to reduce recoil, it was better to use a heavier BCG.  However, those physics guys didn't understand enough about how AR15's work to give the complete answer.

I understand physics pretty well, and I understand AR15's pretty well.  I'm not the MOST experienced at either, but I've been thinking about this a lot lately, and I think I've finally sorted it all out and I'd like to share with you guys :)

Basically, I'm here to report that the 3-gunners were right, you can reduce recoil by using a lighter BCG...but the physics guys were ALSO right..you can also reduce recoil by using a heavier BCG as well!  How can both be right?  Because there are two different methods of achieving recoil reduction, and it depends on how you've built your rifle.  Once you understand the PHYSICS behind your gun's recoil, it will become obvious what I mean...and you'll easily be able to sort out the good advice from the bad advice for your build...so let's get to the physics.

================ PHYSICS INTRO =============

The first law of physics that we must understand is Newton's third law.  It tells us is that if we add up the forces of all the components involved at any slice in time, the sum total will be zero.  It also tells us is that momentum is conserved. Conservation of momentum means that the total amount of momentum in a closed system doesn't change over time.  If we assume the gun and bullet starts with zero momentum (before it is fired), then at any future point in time, the "gun and bullet" still have zero net momentum.

A common mistake when applying conservation of momentum is to forget about the powder or explosive gasses.  For example, I was just looking at a wikipedia article that calculated the momentum of a bullet based on its velocity and mass, and then said the gun must have equal and opposite momentum (recoil).  This is false, because they neglected to consider that there are explosive gasses coming out of the barrel which also have momentum, so the momentum of the gun is more than just the bullet!

============ WALKING THROUGH THE STAGES OF RECOIL ============

Let's walk over what happens in steps over time:

1) The primer ignites the propellant contained in the cartridge case which burns, creating an expanding pressurized gas that pushes the bullet forward in the barrel, and pushes equally hard against the gun itself, creating the primary recoil impulse.

2) When the expanding gas reaches the gas block, some of it is redirected 180 degrees down the gas tube.  In order for the gas to make this 180 degree turn, it has to push off the gas block -- pushing the gun forward.  This counteracts some of the rearward recoil of the gun (and it's not insignificant!).

Tip #1: Opening up your adjustable gas block reduces the force of the latter half of the primary recoil impulse.

3) The bullet exits the muzzle of the gun, and any excess gas that hasn't fully expanding in the barrel goes with it.  If you're using a compensator, some percentage of this gas is reflected back towards the shooter.  When that gas is reflected by the compensator baffles, its basically pushing them forward -- reducing the latter portion of the primary recoil impulse of the gun.  We all know that compensators reduce recoil so this isn't surprising.

4) The gas that was redirected through the gas tube reaches the BCG and pushes it back.  The momentum of this gas is transferred into momentum of the BCG.  The BCG and buffer move together.

Tip #2: Because the BCG and buffer move as a unit, it is the combined weight of BCG and buffer that makes a difference (in general).  If you lose an ounce from the BCG and gain an ounce on the buffer, you really haven't made a difference to anything.  Therefore, we should really talking about the "reciprocating mass" as the BCG + buffer weight.

Tip #3: It is a common misconception to think that when the reciprocating mass starts moving backward, it starts subtracting its momentum from the rearward momentum (recoil) of the gun.  That's false, because the reciprocating mass is merely being pushed by gasses that were ALREADY moving rearward through the gas tube.


Tip #4: Sending back excess gas has some pros and cons.  One of the pros is that you'll see reduced recoil (as per tip #1).  You'll also have more reliable feeding, and faster cycling.  But if it cycles too fast, then it may outpace the feeding of your magazine leading to jams.  Additionally, you'll get more carbon fouling of the gun.


5) The reciprocating mass compresses the buffer spring. The buffer spring can only hold a fixed amount of energy.  If your spring is too weak and isn't able to store all the energy of the reciprocating mass, then it will slam into the back of your buffer tube -- giving any excess energy back to the gun in the form of a secondary recoil impulse.

Tip #5: In order to avoid the reciprocating mass slamming into the back of the buffer tube, the buffer spring should be selected such that it has enough tension to store all the energy of the reciprocating mas.  This depends on the mass of the BCG+Buffer, as well as the velocity, which depends on the gas flow.


6) The buffer spring expands, pushing equally against the BCG and the back of the gun, creating a secondary recoil impulse (or third recoil impulse, if your buffer spring was too weak).

Tip #6: Assuming you aren't planning to excessively over-gas your rifle, the buffer spring tension should be selected primarily such that it can return your BCG to battery in support of your desired cycle rate.  Thus, a higher mass reciprocating mass will require a stronger buffer spring.


Tip #7: Assuming your spring wasn't under-sized, the BCG will return to battery with the same momentum that it had when it went back to begin with.  In other words, any recoil that was absorbed by the reciprocating mass goes right back into the gun with a delay.


Tip #8: If you were over-gassed, any excess gas that slips past the BCG is NOT added back into the recoil of the gun.  That recoil reduction was permanent.

=========== RECAPPING: THE TWO METHODS OF RECOIL REDUCTION ==========

So, to recap, there are really only 2 ways to reduce the primary recoil impulse (without changing ammunition or barrel length):

Approach #1:
Use a compensator to reflect excess gas at the muzzle back at you.  This reduces the primary AND secondary recoil impulses, resulting in a lower final rearward momentum of the gun.  To reduce more recoil, tighten up your adjustable gas block and send more of your gas down the barrel to the compensator!  Of course, this requires reducing mass of your BCG+buffer to still cycle with lower gas pressure, and when you reduce that mass, you also need to reduce buffer spring tension.  Side note: you also get more bullet velocity this way.

Approach #2:
Redirect more gas back into the gun internals.  This can be done either by using a silencer, or by opening up the adjustable gas block.  This reduces the primary recoil impulse.  However, most of that momentum (all except for gas that escapes around the BCG through the ejection port) will just be absorbed by the reciprocating mass (buffer + BCG), and ALL the momentum absorbed by the reciprocating mass will be given back to the rifle in a secondary recoil impulse....so this method does NOT reduce the final rearward momentum of the gun.    Of course, you can't open up the gas block significantly without also increase the weight of your reciprocating mass, or the BCG will move too quickly for reliable cycling....so to use this method, we need to increase weight of the BCG.

Note: any excess gas that escapes out of the ejection port represents a permanent reduction in recoil, and increase in volume, basically using your gun itself like a compensator.

========== TIPS FOR BUILDING YOUR GUN ==========

Everyone seems to agree that it's tough to say exactly which weight buffer, BCG, or spring tension or gas valve level is right...because they all effect each other.  The only thing people can tell you is how to know when something is off, based on the angle of your ejection pattern, etc.  

However, now that we understand the physics, there's a straight forward method you can use to go about selecting all these variables for YOUR gun that is better than just trial and error:

Step 1: The only variable you have complete control over is the reciprocating mass, which is the mass of buffer + BCG.  The relative weight between those two shouldn't really matter much if at all.  Whether or not you want a heavy or a light reciprocating mass is up to you!

Step 2: Once your reciprocating mass is known, a buffer spring should be selected with sufficient tension that it can return the BCG to battery at the desired speed to maintain your desired cyclic rate.  However, unless you have a high speed camera, you likely won't be able to measure that.  So, assuming you can't do this, you'll need to just guess a buffer spring -- using a stiffer spring if your reciprocating mass is greater, and vice versa.

Step 3: Set your adjustable gas pressure using the "fire 1 bullet until the bolt locks back" procedure.  Then look at the ejection pattern.  If your shells don't eject at the proper angle, then it's an indicator you selected the wrong buffer spring tension.  Additionally, you can try sticking a clay ball on the back of your buffer.  If it comes out squished, then you need a stiffer buffer spring..so go back to step #2.

(END)

You know what works great? SJC Titan that's what. Now it is a loud bitch if you are on the side of it. Took what little recoil there is in .223/5.56 and turns it to a .22lr.

TL;DRx8.

J/K.

DR;FI.









Did Read; Found Interesting.

Well written and well stated.  You've identified some important parts of the whole concept of shooting.  And you have accurately identified that plenty of products are pushed with "sales speak" that is anywhere from simplistic to misleading to outright false.

You also have weeded out a lot of "gun counter facts" that are anything but.  Most of those are "what Bubba told me worked for him," and they completely ignore the fact that Bubba could be anywhere from a rocket scientist to a high-functioning moron.  Applying actual science to the issue is incredibly important, because it doesn't matter whether we're talking about either end of the spectrum, the measurable impact of changing rifle factors is the same.

One thing you didn't include is the contribution the ammunition makes to the whole thing.  Mildly loaded 55 grain loads will generate less impulse (both gas effects and "3rd law" effects) than hotter 77 grain loads.  Those 3-gun folks who use light carriers also tweak their loads to generate less impulse, which allows them to tune down the energy of the whole system, so they can stay on target (at quite short ranges) for multiple shots.  Tuning an adjustable gas block is one way to alter impulse, but it's tricky.

I have fixed blocks in all of my "DI" ARs (calling the system "direct impingement" is false, but I'll avoid that discussion right now), and I accept that if I'm firing heavier bullets that there will be a different "feel" to the rifle.  I am a strong "overgassing" cynic; the rifle's designed to operate whether fed relatively wimpy rounds or stout, maximum-load rounds.  I also feel that folks who complain that their rifles "kick too hard" so they "must be overgassed" should fire a Garand a few times...  

The shooter can adapt the rifle to themselves by going with a particular load and carefully tweaking the rifle to run with that, or the shooter can adapt himself to the rifle's characteristics and employ solid marksmanship techniques to be accurate regardless of what load they're shooting.  I place myself in the latter category.

Not correct.

The primer ignites the propellant contained in the cartridge case which burns (not explodes). The cartridge is encased by the chamber of the barrel...

Thanks for your praise, and I'm glad you enjoyed reading it :) Good point about using milder loads to generate lower impulse...that is definitely another method.  I edited my post to say that I was only talking about methods for reducing recoil for the same ammunition and barrel length in this post.  I haven't thought sufficiently about powder burn rates etc to write anything interesting there yet...

A perfectly valid argument.  I don't think one approach is universally better than the other, the important thing is to just understand how the rifle works and what the pros and cons of different approaches will be, then we can all build the rifle the way we like it...and that is the beauty of the AR15.

Thanks, I updated with your correction

I had a very reputable company tell me that I needed to use a h2 style buffer to fix an issue I was having and I did just that and know I am thinking my theory was correct and theirs was wrong as I thought all along.

I started with a carbine weight buffer system, slightly lighter bcg and an adjustable gas block. The gas block seems to be very finicky in that I cant go open more than one setting or closed any settings. You would think a gas block with 16 settings shouldnt be that finicky. So I emailed a few companies and one of them told me that if I remove weight from the BCG I needed to add weight to the buffer to balance it all out and avoid bolt bounce. In my mind I was thinking that doesnt make sense. If I do that it just changes the over all reciprocating mass like OP said. In theory if you want to lighten the BCG you should either keep the buffer the same or lighten it and then adjust the gas block accordingly.

I dont know, but now I have a light weight bcg and an H2 buffer that makes no sense and a still very finicky rifle. Well finicky is probably the wrong word at 80* outside temp it shot 50gr threw 75gr bullets with no isse when tuned. The problem is that it was out pacing the bolt catch when I removed the suppressor but if I tuned it down it would not cycle when I put the suppressor back on.

Finally,  thank for sharing.
 Any chance you could gas length write up?

Recoil reduction is most quickly achieved by having the best grip for the type of shooting being done.

Bad form will completely negate a lot of fancy fine tuning of rifle parts.

Gas pushes the BCG backward, buffer spring pushes it forward...so if you reduce mass of BCG, in theory you need to not only reduce gas but also reduce spring tension.

My guess is that you kept the same buffer spring, and after using a lighter bcg with lower gas, it has significantly less momentum and hence has difficulty overcoming the high tension of the buffer spring...this could result in jams/failure to feed as well as bolt not locking open on last shot.

When the bolt doesn't lock back you would probably think you need to let in more gas, and that might work...but now you your cyclic rate is too fast and outpaces magazine feeding capability giving you even more ftf issues...so any way you adjust the gas is going to cause ftf. Hence, finnicky.

Does this sound like it might be what's going on? Did you try using a yellow reduced tension buffer spring? you might also try cutting down some coils.

How about when firing heavier weapon over a lighter weapon of the same type? Is this what you're talking about with the adding weight to non moving parts? My AR is fairly hefty in weight, but I was wondering if I lighten it up will I get more recoil, or is this just a figment of my imagination?

A lot of good info in this thread.

I have a lightweight BCG, reduced power spring, and 1oz buffer in my 5.56 with adjustable gas block and it runs fine with no cycling issues and has a good ejection pattern.

I've been trying to do the same with my 300BLK build but it keeps having problems loading the next round from the mag.  It runs great with a standard BCG, Tubb lightweight flatwire spring, and H buffer.  When switching to the lighter BCG and buffer is when I have problems.  It seems it still needs the greater mass to strip the next round correctly.

I've found that using a standard weight Tubb flatwire spring with the lightweight BCG and buffer helps strip the next round and get it into battery and function correctly.  Is this normal with some setups when using a lightweight BCG and buffer?

Recoil reduction? Use a 20" rifle.

I don't have lightweight bcg, although still considering getting one,but I run tubbs with taccom buffer and adjustable gas block. Seems to run fairly smooth. I ordered the reduced tension spring with taccom and actually felt like the tubbs felt better with the buffer. I keep hearing a lot of good things about vltor system but not sure how significant improvement it would give

I think your putting a little too much emphasis on the gas pressure in the gas tube pushing the rifle forward.

Part of percieved recoil is based upon cycle time, the more time you spend absorbing the energy from the cycle, the less recoil you will feel. The gentle push vs the fast slap. Same amount of energy dissipated but spread out over much more time.

Increasing rifle weight will lower felt recoil. The rifle has mass, that requires energy to move it. The less it moves, the less you feel recoil. More mass means the rifle reaches a lower peak velocity.

Buffer weight can play a role in cycle speed and dwell time both delaying when the bolt opens and pausing at the rear of the stroke giving the magazine time to feed.

Part of keeping a rifle flat is dealing with the forward impulse of the bcg slamming home. Decreased bcg/buffer weight combined with less spring power lowers the forward impulse.

As mentiones earlier, gas impulse and light loads can make a big difference. 

Something else to consider, if your tuning for reduced spring power and low mass parts, you will need less force if you have less drag when feeding the round. I recommend M3 pmags for their better feed angle. They're also more tolerant to high bolt velocity than most mags, so dropping in a full power load will still feed even if carrier speed is high.

Not bad, however I don't think you've got a great understanding of whats going on in the BCG.

You seem to be treating it a true direct impingement system.While there is certainly a component of direct gas impingement on the carrier, I know from accidental experimentation that there is typically no where near enough gas action to fully cycle the carrier via direct impingement. The gas in directed from the carrier key into carriers internal expansion chamber, as the gas fills the bolt acting as a piston is pressed forward along with the rearward force on the carrier. This expands and unlocks the BCG, at full expansion exhaust ports are exposed by the bolt where the gas can bleed off momentarily, until the gas tube is fully clear of the carrier key, disconnecting the BCG from the gas system as it moves rearward.

Very nice. Thank you OP, interesting read.

Gas in the port has so little effect as to be unnoticeable.

The difference in an open port and sealed barrel can't be felt. It's miniscule.

The gas only effects secondary impulse.

You use a heavy buffer to control an over gassed gun with no adjustabe gas block.

The need for reduced springs withreduced BCG mass is not accurate.

This clearly shows a lack of real world experience with various configurations.

Always one spotlighter. Okay burning guy, it technically deflagrates due to the containment. Primer blast pops bullet forward out of the case and into the throat, where it then stops. During this phase there is a slight and millisecond(s) pressure drop. Now that the barrel is corked, burning powder gases and the flame from the primer blast cause compression heating - which then causes simultaneous ignition or deflagration of the powder granules. They obviously contain an almost stoichiometric ratio of fuel/oxidizer requiring no atmospheric oxygen to burn. The pressure spike which follows, restarts the bullet's movement forward down the barrel, which then starts a race between expanding volume of gas, cooling effect of the barrel, and expanding volume inside the contained space. This is limited by friction, inertia -based on the projectile mass, and the granule size and burning styles of the powder granules. The surface area decreases as the powder granules burns, obviously. 

Pebble, good work, but it looks like you stopped a little short.

> 6) The buffer spring expands, pushing equally against the BCG and the back of the gun, creating a secondary recoil impulse (or third recoil impulse, if your buffer spring was too weak).

That's not the last recoil impulse. After #6, assuming you aren't on an empty mag, the bolt will go home, stripping a new round and eventually closing. When it closes, it transfers forward momentum to the rifle, producing a third (or fourth) recoil impulse. This final impulse is "negative," in the opposite direction of the main impulse, basically causing the rifle to jump forward a bit in your hands/out of the shoulder.

Also, it's clear that one primary goal of setting up a rifle should be to eliminate the BCG bottoming out. This adds a sharp (short duration) spike to the middle of the buffer-spring-related recoil impulse, and doesn't give you any advantage in return. However, you state that this should be avoided by picking a strong enough buffer spring to absorb the full energy of the BCG's rearward motion. Actually, this should be avoided through the tuning of an adjustable gas block. With any choice of buffer spring, it is possible to send only enough gas rearward to fully compress the buffer spring (and therefore allow the bolt to lock open on an empty mag), without bottoming out.

So the real question is, given that we're going to use an adjustable gas block tuned down to the minimum gas required for reliable lockback, what spring and buffer weight should we choose, and why?

Personally I think the logical choice is the lightest possible bcg+buffer weight, paired with whatever spring gives it a desirable cycle time. Choosing the lightest possible mass enables minimization of the recoil impulse from both the compression of the buffer spring and the bolt slamming home (the magnitude of the latter always being half of the former, thanks to conservation of energy and momentum).

I don't think I've ever read a more true statement.

The polymer buffer on the rear of the carrier is supposed to mitigate the carrier’s end travel.  It can only do so much, and while I’ve seen replacement buffers available, I haven't’ seen “a selection of various stiffnesses” in this part.  

So yes, bottoming is a problem, but while you’re looking at that, keep an eye on the condition of the polymer buffer too.  If your poly buffer looks beat up, you’re probably slamming the carrier pretty hard into the rear of the receiver extension.

The recoil buffer in the original AR-15 Model 601, i.e., the Edgewater buffer, weighed less than 2 oz.

IIRC, the original spec ammo for the 601 was slightly less powerful than M193 as well.

It's good to start from the original benchmarks.  That being, the 20" barreled rifle AR-15 or M-16.  Then go to the M4 14.5" carbine.  Filter out all the custom and home builds.

People do too many things which screw up the original specifications and wonder why the cart has fallen off the wheels.  If you're going out of spec, trying to make a lightweight rifle, using the non-62 GR ammo issue-type ammo, lightweight BCGs, changing barrels lengths, gas ports, buffers, springs, etc. don't be expected when the recoil is not what it should be.

Reference the military specification first.  That's how this conversation should begin.

The regular and current 20" M-16A4 should be standard for rifles, followed by the 14.5" M4 for carbines.  They use the appropriate buffers, buffer springs and A2 birdcage flash hider.

From there, your recoil is?

WOW. Very elaborate description of recoil in an AR.

One thing (I think) that should also be factored in is the duration of time the gas is applied and the (pressure) force during this time. Speed vs. energy
In an AR with short dwell time compared to longer dwell time the ''amount of gas introduced'' and the ''duration'' of the gas being introduced changes. The carrier being forced rearward from a shorter more powerful initial movement reacts differently than that of a longer less powerful initial movement even though both end up with an equal amount of overall energy pushing the carrier rearward.

The void in the buffer that allows the weights to slide is a factor when considering the initial force and the time the force is applied.  The location of the weights in the buffer and their effect on the overall weight / resistance being applied at a given time changes as the timing is changed and the overall recoil is then changed.

As the AR recoil forces the gun rearward, the person (or whatever) that is holding the gun and it's location on the gun also effects the recoil. The external forces on the AR effect the timing of the buffer weights and the time that they bottom out to the rear of the buffer.