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Posted: 11/10/2008 5:58:43 PM EST
With all the hype being swallowed about gas piston conversions, I’m convinced most folks don’t understand how the AR works as it left the box. Here’s a half assed attempt to spread a bit of info on how the AR works. The DI AR, does also have a piston… and a cylinder to trap gases to unlock the weapon after a cartridge is fired and unlock the bolt.
The piston is attached to the bottom of the bolt, which fits in the bolt carrier which acts as the gas cylinder(from now referred to as the bolt carrier). The bolt rotates in the cylinder freely, but is limited by the cam pin which is guided in a slot in the bolt carrier.
When the bolt is locked, its cammed rearward and down, when unlocked its forward and upright.
Its all those locking lugs that help maintain a consistent lock up, which is one of the main reasons the AR is as accurate as it is.
The trick to making an auto loading rifle function is getting the bolt to unlock. Most all the early auto loading rifles are unlocked by an operating rod which has a recessed cam area where an extension of the bolt fits. The area is usually shaped the same as the cutout area for the cam pin in the AR bolt carrier. If you have an AK, M1A, M1 Garand, or M1 Carbine I’m sure you know what I’m talking about. When you pull the rod back, the bolt rotates(unlocks). The interesting part is how to use the gas from the spent cartridge to push that operating rod back to unlock the bolt.
In the AR, the operating rod is the gas itself. Instead of a rod in contact with the bolt, gas is diverted from the barrel, down a tube, and directed into the bolt carrier(gas cylinder) directly. When the gas fills the chamber behind the piston it pushes the piston forward (easing unlocking)and the bolt carrier backwards. The piston attached to the bolt stops moving forward when it hits the barrel extension focusing the rest of the gas rearward on the carrier. As the carrier moves backwards, it rotates the bolt by contacting the cam-pin in the groove freeing it to continue rearward until stopped by the recoil spring/buffer.
If you think about it, it’s basically a floating system, which allows for the simple design of the AR. The carrier just moves around in an over sized cylinder. Also – don’t buy into the bullshit about NM carriers being a tighter fit enhancing accuracy, they don’t. The accuracy comes from the consistency of the bolt lock up mentioned above – which has nothing to do with the carrier.
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Anyone who has ever field stripped an auto loading rifle is familiar with a common feature; you didn’t even know the AR doesn’t have or need. All other gas piston operated autoloaders bolts/carriers either ride on grooves milled in the frame, or on rails attached to the receiver of the gun. The AR doesn’t have these, because it doesn’t need them. The Bolt/Carrier system of the AR follows a direct path back and forth which is acted on in line with all the parts. The gas enters from the top, but the pressure is applied inside the cylinder. If you contact the carrier off center it torques/tilts which putts extra stress and wear on the bolt lugs and locking surfaces plus adding stress to parts of the gun that otherwise are not stressed.
Look at all the piston operated auto loaders – the part protruding from the bolt which gets cammed for unlocking are all off center. Without the grooves/rails in the receiver/frame the bolt would be tilting up and down or side to side putting all kinds of wear on the parts and added stress on the bolt and locking lug engagement areas.
I hope this makes sense, I tried to keep it simple. This thread is just about how the AR works. Someone else can start a thread explaining timing, dwell and unlocking issues with the different length barrels and gas port locations. Its not a perfect system. It works better in some applications than others, but all things mechanical are compromises.
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Posted: 11/29/2008 12:05:47 PM EST
[#1]
Outstanding work Steve.
Very few people get to see cutaway drawing and I think you will clear up a lot of peoples misconceptions. |
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IN, USA
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Posted: 11/29/2008 1:19:26 PM EST
[#2]
thanks for taking the time to write this in here.I like the ones you have done, like the stainless barrel wear and trigger.
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Posted: 12/5/2008 7:52:44 PM EST
[#3]
good write up
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Posted: 12/5/2008 10:36:19 PM EST
[#4]
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Posted: 4/16/2009 5:08:30 PM EST
[#5]
So, that brings up a question regarding the AR piston conversions.
Most of these conversions appear to apply pressure to the bolt carrier at the location where they gas tube usually enters. Therefore the pressure to move the carrier back is no longer coming from "in line" internal gas expansion, but from "off center" direct pressure. This "off center" pressure must put stress on the AR BCG and upper that it wasn't originally designed to handle, no? |
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Posted: 4/16/2009 5:59:55 PM EST
[#6]
Originally Posted By Monoz:
This "off center" pressure must put stress on the AR BCG and upper that it wasn't originally designed to handle, no? you are correct |
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Posted: 4/16/2009 6:13:47 PM EST
[#7]
Originally Posted By bigbore:
Originally Posted By Monoz:
This "off center" pressure must put stress on the AR BCG and upper that it wasn't originally designed to handle, no? you are correct That's the whole "carrier tilt" issue with piston ARs. The AR15 upper receiver and bolt carrier aren't designed for and don't have a provision to properly control that motion. |
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Posted: 4/16/2009 11:37:51 PM EST
[#8]
Originally Posted By Gamma762:
That's the whole "carrier tilt" issue with piston ARs. The AR15 upper receiver and bolt carrier aren't designed for and don't have a provision to properly control that motion. I was completely ignorant of this - googling "carrier tilt" has started to clue me in. |
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Posted: 4/18/2009 10:54:05 AM EST
[#9]
Fantastic write up, and it really shows the genius of the design.
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Posted: 7/8/2009 10:10:19 PM EST
[#10]
im all about this write up.
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TX, USA
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Posted: 7/21/2009 11:05:25 PM EST
[#11]
The aspect of the piston in an AR making the BCG tilt makes sense. Ive seen some photos of this wear on the front part of the buffer tube. Thats enough for me.
Im glad I found this thread as I was about to drop some bucks on a new piston carbine. Now I think I will just put the funds toward more ammo, which is what I really wanted to do anyway. 
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Posted: 9/8/2009 1:38:28 AM EST
[#12]
Originally Posted By Gamma762:
Originally Posted By bigbore:
Originally Posted By Monoz:
This "off center" pressure must put stress on the AR BCG and upper that it wasn't originally designed to handle, no? you are correct That's the whole "carrier tilt" issue with piston ARs. The AR15 upper receiver and bolt carrier aren't designed for and don't have a provision to properly control that motion. Carrier Tilt = Chewed up buffer tube (You can use a new anti tilt buffer and/or extend the buffer tube into the upper enough that the carrier rests on it in the locked position) Cam pin drag = Gouge in upper from cam pin digging into it (You can add a spring on the bolt tail that pushes at the carrier from inside and/or use a Roller bearing cam pin from POF) As far as a piston upper being better with suppressor use, that's total bunk. The Piston guns get as dirty as the DI guns. 
As for gas in your eye just use a little gasket sealant on your charging handle. |
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Posted: 6/17/2010 12:39:36 PM EST
[#13]
tagggg
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Posted: 6/17/2010 1:49:38 PM EST
[#14]
There is another benefit to the original DI system, that being the bolt is unloaded by the rearward motion of the carrier+buffer when the gas enters the cylinder. Newton's Third Law, every action has an equal and opposite reaction. This isn't trivial, it is 3000-8000 pounds of force which happens shortly after the bullet passes the port. Because of this forward force on the bolt, the locking lugs are unloaded, reducing wear and tear on these surfaces. Wear on these surfaces is the primary mode of headspace growth.
The brass cartridge case expands as a result of the pressure rise but because of spring back, it doesn't fully expand to fill the chamber. But spring back is less than thermal expansion which WILL cause the case to fully load the bolt's lugs until the case cools. This isn't a problem with bolt action rifles but is is with semi-autos. Cycle any bolt gun as fast as you can after firing, catching the brass. It will be barely warm. Not so in a semi-auto, brass from a 5.56 can cause skin burns. |
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Posted: 6/17/2010 1:53:40 PM EST
[#15]
Thank you for this. Very helpful and informative.
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Posted: 6/17/2010 2:07:05 PM EST
[#16]
Originally Posted By SOC: Originally Posted By Gamma762: Originally Posted By bigbore: Originally Posted By Monoz: This "off center" pressure must put stress on the AR BCG and upper that it wasn't originally designed to handle, no? you are correct That's the whole "carrier tilt" issue with piston ARs. The AR15 upper receiver and bolt carrier aren't designed for and don't have a provision to properly control that motion. Carrier Tilt = Chewed up buffer tube (You can use a new anti tilt buffer and/or extend the buffer tube into the upper enough that the carrier rests on it in the locked position) Cam pin drag = Gouge in upper from cam pin digging into it (You can add a spring on the bolt tail that pushes at the carrier from inside and/or use a Roller bearing cam pin from POF) As far as a piston upper being better with suppressor use, that's total bunk. The Piston guns get as dirty as the DI guns. http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548680695-2d8feb0d23-b-tm1.jpg http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548689487-8449978d0e-b-tm.jpg http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548691655-4ae7aa2caf-b-tm1.jpg http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548693999-efaf0fe1d3-b-tm.jpg As for gas in your eye just use a little gasket sealant on your charging handle. That's from the 416 upper and the guy shot an enormous amount of ammo through it. I'm not a DI hater but a DI AR would look like a charcoal briquette after the same treatment. |
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Posted: 6/17/2010 6:01:49 PM EST
[#17]
Anyone have a link to that thread?
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Posted: 6/17/2010 6:43:15 PM EST
[Last Edit: Gamma762]
[#18]
Originally Posted By Keith_J:
Because of this forward force on the bolt, the locking lugs are unloaded, reducing wear and tear on these surfaces. Wear on these surfaces is the primary mode of headspace growth. I've also had the thought for a while now that this mechanism not being correctly in time is why carbine gas 14.5 & 16" barrel ARs have a tendency to shear the locking lugs next to the extractor. Those lugs have nearly no lateral support, and in those gas system configurations the bolt is still under rearward force when unlocking starts, putting lateral stress on the lugs. I've come to believe that forward push as a correctly applied balancing force during unlocking is an important part of the operating cycle and one of the most genius aspects of the design. |
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Posted: 6/17/2010 8:34:02 PM EST
[#19]
Has anyone removed a barrel extension on a piston gun to see the inside locking lug wear and bolt lug wear pattern compared to DI?
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FL, USA
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Posted: 6/17/2010 9:11:30 PM EST
[#20]
And in spite of all that, mine wouldn't function reliably with Wolf ammo.
Long live the AK! |
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Posted: 6/18/2010 12:02:31 AM EST
[Last Edit: TexasRifleman1985]
[#21]
Originally Posted By amd_dude: Originally Posted By SOC: Originally Posted By Gamma762: Originally Posted By bigbore: Originally Posted By Monoz: This "off center" pressure must put stress on the AR BCG and upper that it wasn't originally designed to handle, no? you are correct That's the whole "carrier tilt" issue with piston ARs. The AR15 upper receiver and bolt carrier aren't designed for and don't have a provision to properly control that motion. Carrier Tilt = Chewed up buffer tube (You can use a new anti tilt buffer and/or extend the buffer tube into the upper enough that the carrier rests on it in the locked position) Cam pin drag = Gouge in upper from cam pin digging into it (You can add a spring on the bolt tail that pushes at the carrier from inside and/or use a Roller bearing cam pin from POF) As far as a piston upper being better with suppressor use, that's total bunk. The Piston guns get as dirty as the DI guns. http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548680695-2d8feb0d23-b-tm1.jpg http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548689487-8449978d0e-b-tm.jpg http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548691655-4ae7aa2caf-b-tm1.jpg http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548693999-efaf0fe1d3-b-tm.jpg As for gas in your eye just use a little gasket sealant on your charging handle. That's from the 416 upper and the guy shot an enormous amount of ammo through it. I'm not a DI hater but a DI AR would look like a charcoal briquette after the same treatment. I've run my ARs until they look like charcoal briquettes on the inside... And they still run.  Keep them very wet and the carbon fouling will clean itself off of the important contact surfaces, allowing it to continue functioning... Virtually indefinitely. Two years of heavy training (over 30K rounds per year) on one of my older rifles without a single non-magazine caused malfunction, never cleaned it in the slightest, just kept it dripping wet. I have more than enough confidence in the system. I am, however, a fan of OCD cleaning of all my go-to weapons, not because they require it for function, but because it allows you to intimately inspect the weapon for parts wear or failures. Pay particular attention to the bolt on the AR, it does tend to wear out somewhat quickly. The under-engineered cam pin hole and cam pin is my only complaint on the system. A good DI gun still runs longer without a mechanical failure than any piston-fucked AR in my experience. Pistol guns break, from what I've seen. TR85. |
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Posted: 6/18/2010 8:14:09 PM EST
[#22]
I went with a piston after having problems with my 6.8 DI. After two gas block changes and all the associated fooling around with pins, alignment, leak checks and absolutely mysterious problems, I'm hoping to see an improvement. Won't know until I go to the range, but if it goes bang when I pull the trigger and feeds the next round, I'll be happy.
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AZ, USA
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Posted: 6/19/2010 3:23:32 PM EST
[#23]
So how does this affect other calibers since the DI system was designed around the 5.56mm? Say 6.8mm or the big guy I'm interested in the 8.6mm (.338 LM)? The latter I've only seen in piston form. I'm a DI guy but I don't know if the larger cartridge and pressures will necessitate a piston or not.
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Posted: 6/19/2010 4:56:00 PM EST
[#24]
Originally Posted By TexasRifleman1985:
Originally Posted By amd_dude:
Originally Posted By SOC:
Originally Posted By Gamma762:
Originally Posted By bigbore:
Originally Posted By Monoz:
This "off center" pressure must put stress on the AR BCG and upper that it wasn't originally designed to handle, no? you are correct That's the whole "carrier tilt" issue with piston ARs. The AR15 upper receiver and bolt carrier aren't designed for and don't have a provision to properly control that motion. Carrier Tilt = Chewed up buffer tube (You can use a new anti tilt buffer and/or extend the buffer tube into the upper enough that the carrier rests on it in the locked position) Cam pin drag = Gouge in upper from cam pin digging into it (You can add a spring on the bolt tail that pushes at the carrier from inside and/or use a Roller bearing cam pin from POF) As far as a piston upper being better with suppressor use, that's total bunk. The Piston guns get as dirty as the DI guns. http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548680695-2d8feb0d23-b-tm1.jpg http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548689487-8449978d0e-b-tm.jpg http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548691655-4ae7aa2caf-b-tm1.jpg http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548693999-efaf0fe1d3-b-tm.jpg As for gas in your eye just use a little gasket sealant on your charging handle. That's from the 416 upper and the guy shot an enormous amount of ammo through it. I'm not a DI hater but a DI AR would look like a charcoal briquette after the same treatment. I've run my ARs until they look like charcoal briquettes on the inside... And they still run.
Keep them very wet and the carbon fouling will clean itself off of the important contact surfaces, allowing it to continue functioning... Virtually indefinitely. Two years of heavy training (over 30K rounds per year) on one of my older rifles without a single non-magazine caused malfunction, never cleaned it in the slightest, just kept it dripping wet. I have more than enough confidence in the system. I am, however, a fan of OCD cleaning of all my go-to weapons, not because they require it for function, but because it allows you to intimately inspect the weapon for parts wear or failures. Pay particular attention to the bolt on the AR, it does tend to wear out somewhat quickly. The under-engineered cam pin hole and cam pin is my only complaint on the system. A good DI gun still runs longer without a mechanical failure than any piston-fucked AR in my experience. Pistol guns break, from what I've seen. TR85. Piston guns are more tollerant of debris and lack of cleaning. This was clearly proven in the Dust test. The DI guns failed badly when exposed to sand and dust while the piston guns ran longer. That isn't to say DI guns suck...but pistons have their place. |
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Posted: 6/23/2010 8:00:06 AM EST
[#25]
Thanks for the informative writeup. Very well written.
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Posted: 2/6/2011 6:47:26 AM EST
[#26]
Fantastic explanation, just saw it another thread here. Great stuff. Thanks.
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OH, USA
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Posted: 1/14/2012 3:11:24 PM EST
[Last Edit: curmudgeon10]
[#27]
bigbore: Thanks a lot for showing/telling how my AR-15 works internally; clears up many questions that were rattling around in my brain.
Fantastic !!!
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Posted: 1/22/2012 6:34:39 PM EST
[Last Edit: sinlessorrow]
[#28]
Originally Posted By LockingBlock:
Originally Posted By TexasRifleman1985:
Originally Posted By amd_dude:
Originally Posted By SOC:
Originally Posted By Gamma762:
Originally Posted By bigbore:
Originally Posted By Monoz:
This "off center" pressure must put stress on the AR BCG and upper that it wasn't originally designed to handle, no? you are correct That's the whole "carrier tilt" issue with piston ARs. The AR15 upper receiver and bolt carrier aren't designed for and don't have a provision to properly control that motion. Carrier Tilt = Chewed up buffer tube (You can use a new anti tilt buffer and/or extend the buffer tube into the upper enough that the carrier rests on it in the locked position) Cam pin drag = Gouge in upper from cam pin digging into it (You can add a spring on the bolt tail that pushes at the carrier from inside and/or use a Roller bearing cam pin from POF) As far as a piston upper being better with suppressor use, that's total bunk. The Piston guns get as dirty as the DI guns. http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548680695-2d8feb0d23-b-tm1.jpg http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548689487-8449978d0e-b-tm.jpg http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548691655-4ae7aa2caf-b-tm1.jpg http://www.thefirearmblog.com/blog/wp-content/uploads/2009/06/3548693999-efaf0fe1d3-b-tm.jpg As for gas in your eye just use a little gasket sealant on your charging handle. That's from the 416 upper and the guy shot an enormous amount of ammo through it. I'm not a DI hater but a DI AR would look like a charcoal briquette after the same treatment. I've run my ARs until they look like charcoal briquettes on the inside... And they still run.
Keep them very wet and the carbon fouling will clean itself off of the important contact surfaces, allowing it to continue functioning... Virtually indefinitely. Two years of heavy training (over 30K rounds per year) on one of my older rifles without a single non-magazine caused malfunction, never cleaned it in the slightest, just kept it dripping wet. I have more than enough confidence in the system. I am, however, a fan of OCD cleaning of all my go-to weapons, not because they require it for function, but because it allows you to intimately inspect the weapon for parts wear or failures. Pay particular attention to the bolt on the AR, it does tend to wear out somewhat quickly. The under-engineered cam pin hole and cam pin is my only complaint on the system. A good DI gun still runs longer without a mechanical failure than any piston-fucked AR in my experience. Pistol guns break, from what I've seen. TR85. Piston guns are more tollerant of debris and lack of cleaning. This was clearly proven in the Dust test. The DI guns failed badly when exposed to sand and dust while the piston guns ran longer. That isn't to say DI guns suck...but pistons have their place. those tests were set up poorly, if you look the summer testing showed the DI rifle only having 148 class 1&2 stoppages seen here 
now the issue with the fall testing done, is that the trigger pullers didnt realize how burst mode worked, so every time they only got 1 or 2 shots they counted that as a stoppage, even though it wasnt, and its how the trigger group is setup. after the fall dust test colt hired an outside DOD-certified testing agency, Stork East-West Technology Corporation in Jupiter, Florida to do the exact same dust test and came out with 111 class 1&2 stoppages. so the fall dust test was a horribly setup testing, and most of those stoppages are from not firing a 3-rnd burst fully. |
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Posted: 3/26/2013 8:44:46 AM EST
[#29]
I love the guys at ADCO.
This is a very good read.  |
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