Suppressed Cycling Issues with AR-15

Is it short stroking....check to make sure the correct buffer and spring are being used...

And....NO.....I'm not an expert...

First what caliber is it?
How many rounds have been fired through it?
Does it work without the suppressor?
Are you running the action wet?
Does the bolt lock open on the last shot?
Have you tried different magazines, are the mags the right ones for the caliber?


SC45-70

More details on the rifle in question would help. That said usually when a suppressed rifle that is otherwise functional (IE no bad or damaged parts) then cycling issues are most often the result of the bolt moving too fast. The gas system is doing its job normally but the suppressor is keep barrel pressure higher longer and once the bolt unlock the case in the chamber act like a pistol throwing the bolt even faster to the rear. This can cause issue include bolt over base were the rounds in the magazine do not rise fast enough to be up in the right location before the bolt/buffer bounce of the back of the receiver extension and go over the round in the magazine rather than stripping them from the magazine.

The best solution is usually some type of adjustable gas block that will let you turn down the gas system when running suppressed.

Another possible solution is to go to a heavier buffer. If the bolt velocity is only a little over normal this might solve it.

What cartridge?
What length barrel and gas type and length system?
What suppressor?
Does it run without the suppressor?
Where are the ejected case going? Forward? Back?

Are you getting gas in the face when shooting with the suppressor on the gun? I have a number of AR's that take my suppressor, and pretty much all of them will gas you out if you use the suppressor and dont Permatex the charging handle. Dont get that without the suppressor.

They all run fine without the can on the gun, and they run fine with it on the gun, but you can feel it run a bit harder and the ejection pattern is different with it on there. So Id say in my case, its probably a bit over-gassed with the suppressor on there, or at least it seems that way to me.

Might sound silly, but what kind of stock is on the gun? The problems you're describing is something like what I was running into with an A1 stock on the gun, with an A2 screw holding it on.

marksman13 said:

When fired, the rifle will eject the spent casing pretty reliably, but fails to pick up the next round from the mag more often than not. Usually, the rifle will fire two consecutive rounds before failing to pick up the third round from the mag.

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Does the BCG lock back if the Mag is empty or close?

marksman13 said:

Yes. Bolt locks to the rear on an empty mag.

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I would try running it without a can and see if it likes that. If it does, I’d throw an adjustable gas block on it and cut it down until it runs right note position, keep going until it quits running because there is not enough gas to operate and put adjustment in the middle of the two points.

Like @mcb said, most likely over-gassed and bolt is outrunning the magazine.

I have an 18" semi auto .308 and a 13" post sample MG, both mid length gas. The 18 incher would always outrun the mag with fixed gas. The 13" machine gun has a shorter dwell time and did OK semi-auto, but would still malfunction in full auto. Odin works AGB on the 18, drilled & tapped the FSB for an adjustment screw on the MG, problem solved.

5.56 guns tend to be more forgiving with cans, but you can still lower port noise and calm down ejection with gas adjustment.

marksman13 said:

Quick question for the experts, but first let me set up the scenario.

I have a rifle. We’ll call it an AR-15 at the moment, but it is not chambered in 5.56. This rifle has only been run suppressed. It has always been run with quality, supersonic ammo. It has NEVER run reliably. When fired, the rifle will eject the spent casing pretty reliably, but fails to pick up the next round from the mag more often than not. Usually, the rifle will fire two consecutive rounds before failing to pick up the third round from the mag. This has been consistent across four brands of ammo and with Magpul and Hexmag magazines.

My initial impression was that the rifle was severely under gassed. I thought that the gas block was probably not properly aligned with the gas port and that was restricting gas flow. Normally when shooting suppressed the issue is over-gassing in my experience, which I have not observed affecting operation, only excessive wear and tear on the moving parts.

Could over-gassing be causing the issue I described above? Could it somehow be cycling to BCG so fast that it is not picking up a round from the magazine? As in, the action is cycling faster than the spring can push the next round to the top of the mag. Maybe the top round in the mag has enough spring tension behind it to keep it at the top of the mag, but the following rounds don’t? I’m completely spit-balling on this one and hoping some of you guys can help me out here.

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You say suppresed, but my question is what all has been changed from factory, did you change the gas block?

Well the ar platform is a "blowback" , meaning some of the barrel pressure is diverted into the gas block and is used to drive the bolt back. if you are short cycling this may be an issue.
The supressor is limiting the amount of gases and burnt powder allowed to exit the muzzle.

(Only a small portion of pressure is used for the gas block, the rest is blown out the muzzle)

"You have essentally added some restriction with a supressor"

I have had several friends have this same problem and the culprit was a clogged gas block hole.

45CalPal said:

Well the ar platform is a "blowback" , meaning some of the barrel pressure is diverted into the gas block and is used to drive the bolt back. if you are short cycling this may be an issue.
The supressor is limiting the amount of gases and burnt powder allowed to exit the muzzle.

(Only a small portion of pressure is used for the gas block, the rest is blown out the muzzle)

"You have essentally added some restriction with a supressor"

I have had several friends have this same problem and the culprit was a clogged gas block hole.

Click to expand...

No offense but that does not make sense! IF the suppressor limits or delays the gas ejection from what the large exit hole of an open barrel would have THEN that pressure will tend to be higher and longer dwell at the gas port and will cause the higher bolt velocity with the mag overrunning problems described above. This is a well known problem on the AR format and why Quality manufacturers who were aware of it provide "Switch Block " two position gas blocks that reduce down the gas in suppressed use , or people put on adjustable gas blocks. Yes there are miss drilled gas ports or mis aligned gas blocks that inhibit gun function un suppressed by undergassing. A few oversize gas ports are around that inhibit good function un suppressed (and worse suppressed) but standard sized acccording to barrel and gas tube length, well positioned gas systems , a lot of the time need adjustable gas blocks . There are Bolt Carriers out their which are gas adjustable too,but I wouldn't use one.

45CalPal said:

Well the ar platform is a "blowback"

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No.

You misunderstand autoloading firearm operating system terminology.

Blowback guns do not have a locked breech and do not bleed gas from the bore to function.

Some delayed blowback systems use gas pressure from the bore to delay breech opening, but that is not how the AR works.

The AR is a gas operated rifle with a unique system using the bolt itself as a piston.

That covered, suppressors increase bolt velocity on gas operated firearms because they protract the gas impulse, increase dwell time.

marksman13 said:

Yes. Bolt locks to the rear on an empty mag.

Click to expand...

Then it's not undergassed!

While unusual, all the symptoms you describe can be explained by excess bolt speed, aka overgassing.

MachIVshooter said:

No.

You misunderstand autoloading firearm operating system terminology.

Blowback guns do not have a locked breech and do not bleed gas from the bore to function.

Some delayed blowback systems use gas pressure from the bore to delay breech opening, but that is not how the AR works.

The AR is a gas operated rifle with a unique system using the bolt itself as a piston.

That covered, suppressors increase bolt velocity on gas operated firearms because they protract the gas impulse, increase dwell time.

Click to expand...

With the terminology you have me at, I will agree,

However the functionality of the gas system and an addition of the supressor will in fact slow the release of the pressureized gas within the barrel,
(It's called added restriction) this in turn will create more time for "accumilated" powder build up within the gas system during each cycle.

The tiny hole in the barrel and the gas block will in turn become restricted as a result. that's fact,

Anything involving a tube (barrel) and air flow (Gasses) will have an altered flow rate when the tube entrance and/or the exit of said tube is being altered from the original factory componets. Such as adding a supressor. this is adding actual restriction in flow rate and possibly timming since it functiones off of this gas.

Example:
I work with small air passages every day for the past 25+ years on engines, I was at one point ASE and GM certified So I have studied the folowing:
What I speak of is in "similar" comparison to that of the intake system of an engine or the exaust system. when the air filter is removed the flow rate is increased, same as any other changes to the air intake system will result in changes down the line. Or the muffler being removed allows a more free flow, and in turn alters the "tuning" velocities of the gasses. both of these changes can and will alter the runability of the engine .

So by adding the supressor, the timming of the gas exit has changed (it is now Slower), the flow rate has changed (air flow velocity) etc... You are however correct in the slowed gas exit has an effect of increasing the pressure within the barrel and this increased pressure will result in higher exiting velocities of projectiles.
But this all has altered the OEM function that the hole diameter was based on, and you now have a higher and more likely chance of restricting the flow rates weather within the gas system or the barrel exit.

I will also add that the increased pressure within the barrel can and will effect the "movement" of air (gases) as well as the temprature of said gasses.
Fact: the molecules of hot air will move faster than the moleules of cold air

Now, granted the minor change of this restriction is a minor amount as one might expect, but the system such as the gas hole diameter in the barrel, the hole diameter in the gas block, and the inner diameter of the gas tube are essentially a "Tuned System" from the factory. So when you start to alter one or the other all componets in the chain are in turn effected. Same result as adding an adjustable gas block, the bolt can get over gassed or under gassed and chain reacts into failure to feed issues and/or eject. The entire function of this gun is dependant upon the correct amount of gas flow and pressure.

furthermore, the increased pressure has now allowed more burnt powder debris to be able to be pressurized past the chamber and can now increase the amount of build up on the bolt and extractor. (The Chain Reaction)

So in conclusion , to the OP... yes the supressor has in fact altered the function of the gas system, if all you have done is the supressor, I recomend you disassemble the gun and give it a good cleaning, possibly install an adjustable gas block to be able to compensate for the restriction.
it would make sense to increase the barrel gas hole diameter for this adjustability to be more functional and acurate. I believe your running "under gassed".

45CalPal said:

With the terminology you have me at, I will agree,

However the functionality of the gas system and an addition of the supressor will in fact slow the release of the pressureized gas within the barrel,
(It's called added restriction) this in turn will create more time for "accumilated" powder build up within the gas system during each cycle.

The tiny hole in the barrel and the gas block will in turn become restricted as a result. that's fact,

Anything involving a tube (barrel) and air flow (Gasses) will have an altered flow rate when the tube entrance and/or the exit of said tube is being altered from the original factory componets. Such as adding a supressor. this is adding actual restriction in flow rate and possibly timming since it functiones off of this gas.

Example:
I work with small air passages every day for the past 25+ years on engines, I was at one point ASE and GM certified So I have studied the folowing:
What I speak of is in "similar" comparison to that of the intake system of an engine or the exaust system. when the air filter is removed the flow rate is increased, same as any other changes to the air intake system will result in changes down the line. Or the muffler being removed allows a more free flow, and in turn alters the "tuning" velocities of the gasses. both of these changes can and will alter the runability of the engine .

So by adding the supressor, the timming of the gas exit has changed (it is now Slower), the flow rate has changed (air flow velocity) etc... You are however correct in the slowed gas exit has an effect of increasing the pressure within the barrel and this increased pressure will result in higher exiting velocities of projectiles.
But this all has altered the OEM function that the hole diameter was based on, and you now have a higher and more likely chance of restricting the flow rates weather within the gas system or the barrel exit.

I will also add that the increased pressure within the barrel can and will effect the "movement" of air (gases) as well as the temprature of said gasses.
Fact: the molecules of hot air will move faster than the moleules of cold air

Now, granted the minor change of this restriction is a minor amount as one might expect, but the system such as the gas hole diameter in the barrel, the hole diameter in the gas block, and the inner diameter of the gas tube are essentially a "Tuned System" from the factory. So when you start to alter one or the other all componets in the chain are in turn effected. Same result as adding an adjustable gas block, the bolt can get over gassed or under gassed and chain reacts into failure to feed issues and/or eject. The entire function of this gun is dependant upon the correct amount of gas flow and pressure.

furthermore, the increased pressure has now allowed more burnt powder debris to be able to be pressurized past the chamber and can now increase the amount of build up on the bolt and extractor. (The Chain Reaction)

So in conclusion , to the OP... yes the supressor has in fact altered the function of the gas system, if all you have done is the supressor, I recomend you disassemble the gun and give it a good cleaning, possibly install an adjustable gas block to be able to compensate for the restriction.
it would make sense to increase the barrel gas hole diameter for this adjustability to be more functional and acurate. I believe your running "under gassed".

Click to expand...

Yeah............that's a lot more nope.

Before I went full time with my current company manufacturing & repairing suppressors and firearms, I spent 21 years turning wrenches for a living. The parallels that can be drawn between firearm silencers and internal combustion engine exhaust systems & mufflers begin and end with reducing sound pressure and altering frequency. IC engines don't use exhaust gasses to facilitate a mechanical cycle in the engine; the exhaust gas impulses do not actuate the valves or pulse the injectors. The EGR system is the only place exhaust gas is routed back to the engine, and that's in no way similar to any species of gas operated firearm. Really can't even liken it to a.turbocharger, just totally different animals. IC engine exhaust also has nowhere near the pressure and temperature of gunpowder burning in a barrel , nor are you dealing with supersonic gasses with engine exhaust. The fluid dynamics are completely different.

As well, gas port or tube clogging in the AR is just a non-issue. Ports may erode and tubes sometimes melt on machine guns, but they don't clog.

The ick that winds up in the receiver also doesn't come from the gas tube. It's a result of increased residual pressure in the bore after the breech has opened. Blowback and recoil operated firearms experience the same increase in debris and fouling with a suppressor. Only manual action firearms stay clean with a can.

The OP's weapon is overgassed and outrunning the magazine due to the higher bolt velocity caused by the increased dwell time the suppressor creates. This is a well known issue with suppressing gas operated rifles and machine guns.

What the man said above ! I might add with modern correct powders in the 5.56 round being discussed it was found a very long time ago that not very much "debris" builds up in the direct impingement (correct term for the system) gas tube or port or block with 50,000 psi blowing thru it at hypersonic speeds.
And yes there are not that much similarities between engines and firearms, as much as us mechanics (or mechanical engineers that designed engines for Detroit diesel) want to think there is.

45CalPal said:

…possibly install an adjustable gas block to be able to compensate for the restriction.
it would make sense to increase the barrel gas hole diameter for this adjustability to be more functional and acurate. I believe your running "under gassed".

Click to expand...

An adjustable gas block isn’t going to make an undergassed gun run, unless it is accompanied by a larger diameter hole in the barrel.

A suppressor is in effect a muffler, using your auto analogy, it creates back pressure when the “exhaust” isn’t allowed to leave as freely as it can without it in place, more pressure is going to find itself on the end opposite the restriction. This is why things like “gas buster” charging handles exist when unsuppressed (properly gassed) rifles don’t really need them.

The questions like, does the BCG lock open or rifle run with/without a suppressor are the keys to knowing what side of working/not working because of too much or not enough pressure you are on.

You need a large enough port diameter (hole in barrel) to function with the load you will use that has the least amount of pressure. If the action can’t go all the way back, it is under gassed, too heavy or sprung.

If the BCG is moving rearward far enough to get caught by the catch after the last round, that’s enough gas to push the BCG, buffer, against spring, far enough to strip another round from the magazine. If it can’t do that, that’s under gassed. If it not only can but do so with so much force it shoves the BCG, buffer, into the springs to coil bind and bounce back before the magazine spring has time to feed the next round in the column to the feed lips, you are over gassed.

If the port is too large (over gassed) or too light a buffer or under sprung and you need to reduce the gas assistance further, that is the intent of an adjustable gas block. You can’t get more gas out of the hole in the barrel than “open”, adjustable gas blocks can only make that amount smaller.

Nice to have a happy ending. Glad you got it figured out.

When you set it up, I would do so like posted in #9. If you are going to run it unsuppressed too, I would test it with and without the suppressor. The best spot will be in the middle of the overlap of the two “running” ranges.