Feeding issue while suppressed (AR-15)

[tmorone]

So I had a really interesting experience at the range this morning, and it made me wonder if anyone here has ever experienced the same thing...

I was running a 14.5" Daniel Defense barrel with an AAC M4-2000 suppressor, H buffer, and Magpul P-Mag. About every third shot the carrier group would come forward and get jammed up about 1/2" from being completely into battery. After extracting the round, every time the case would be seriously dented and scratched from the bolt lugs digging into it.

At first I was thinking maybe the gun was over-gassed, or dirty. Then I switched to a Surefire-60 Magazine and the problem went away. Different P-mag, still ran like a champ.

Unsuppressed, the rifle runs great also- even with problem magazine #1.

Question is, could it really be that magazine causing the issue? It seems so weird that it's fine until I put the can on it, and then only that one particular mag jams things up. I've thought about trying an H2 buffer to slow the action down, but worry about losing reliability unsuppressed if I take that route. Or it's a common issue and I'm just reading waaay too far into this

 

[Creeper]

the carrier group would come forward and get jammed up about 1/2" from being completely into battery. After extracting the round, every time the case would be seriously dented and scratched from the bolt lugs digging into it.

I recall an issue like yours in the AR15.com forums. Some respondents suggested trying H through H3 buffers, while others suggested H2 & 3 only... and others suggested that it was ammo issues.

Ultimately, the OP removed the Larue m16 BCG and swapped in a S&W M&P 15 BCG and it "worked flawlessly suppressed and unsuppressed".
There was a lot of conjecture as to what was different between the two BCGs, but the only stand-out variation (according to the OP) was that the S&W BCG was 2 ounces heavier than the Larue.

C

 

[tmorone]

I've got about 10 cartridges that look EXACTLY like that! Thank you so much, I'll try out the next heavier buffer then. Still blows my mind that it only happened with one magazine- yet doesn't seem like a 'normal' magazine induced feeding issue.

 

[Creeper]

I've got about 10 cartridges that look EXACTLY like that!

You and about 10 thousand other guys.

Honestly... there is no easy, one size fits all cure. BCG weight and mechanical resistance, magazines, gas pressure and recoil spring pressure... any and all can be contributors. I have many decades of AR "experiences", but I can't, with any easy certainty, diagnose your problem... only offer possibilities.

If you do a "suppressed AR15 FTF" search on the internet, you'll find lots of valid links with possible causes. That photo came from such a AR15.com thread.

Good luck sorting it,
C

 

[David Hineline]

The bolt/carrier group is cycling too fast for the magazine to feed, heavier buffer, heavier recoil spring, or adjustable gas system.

 

[tmorone]

As a follow up...

I picked up an H2 buffer and swapped it for the H that was installed when the problem first began. Unfortunately, problem still exists and seems like the buffer didn't improve it in the slightest.

Just ordered one of the Wolff extra power springs and fingers crossed that it solves the problem. In a perfect world I'd put a switchblock on the the rifle and call it a day, but it's pinned/welded and has a 12" rail on a 14.5" barrel- so that would be absolute worst case scenario.

Thanks for the help and advice guys, I appreciate it.

 

[Powderman]

Honestly... there is no easy, one size fits all cure. BCG weight and mechanical resistance, magazines, gas pressure and recoil spring pressure... any and all can be contributors. I have many decades of AR "experiences", but I can't, with any easy certainty, diagnose your problem... only offer possibilities.

Actually, there is a solution...but it requires professional help.

A quick illustration:

We all are aware of how the timing in the AR series of rifles works. The cartridge is fired; powder burns as the bullet proceeds down the bore. As the bullet passes the gas port, gas starts down the tube, and heads back to the carrier key to enter the back of the carrier and start the unlocking process. Right about the time the gas gets to the carrier key, the bullet leaves the barrel. Just in the instant before the carrier unlocks, pressure has dropped to a safe level, and the rifle completes its cycle of operation.

Now, let's throw in the monkey wrench--the suppressor.

Same thing happens--the rifle fires, gas moves the bullet forward, the bullet passes the ejection port and the cycle of operation continues.

Here's the difference: In the regular rifle, the bullet leaves the muzzle and the gas escapes.

In the suppressed rifle, the gas is released--but then it encounters a new problem: The blast baffle, aka the first baffle in the suppressor. The bulet passes through, again greatly restricting the gas flow for a microsecond.

By this time, the gun is unlocking. The bullet continues its journey--but a large quantity is still in the suppressor. It HAS to go somewhere...but wait!

Another gas tube has opened up, while there is still pressure in the suppressor. That gas tube is the rifle bore--and gas goes roaring back down the barrel. The gas is still under a lot of pressure.

One indication is to look at the top of the disconnector. The hammer will slam it with so much force that a line of clean metal will be very visible. It will bend pins and break disconnectors if this continues.

Solution? There are two. One is to have a special barrel installed with a smaller gas port to decrease the gas flow.

Or, you can have an adjustable gas block installed.

 

[David Hineline]

There are more solutions than two that deal with reducing gas.

Increasing mass of the moving weight of the parts that cycle or increasing spring tension that fights the movement of the mass.

Increase the mass of the group too much or spring tension too much and the gun will not operate without the silencer.

Increase the spring tension too much without increasing mass of the group can result in cycle speeds in the closing direction too fast the a magazine can not feed fast enough and bolt carrier bounce open can be excessive.

Get the mix just right and there is not need for adjustable gas system.

 

[tmorone]

To follow up...

I finally got some time to order a few new parts and get out to the range. Thought I would post a final reply of what actually worked for me in case someone else is searching about the issue and could use this.

After the H2 buffer AND Wolff XP spring didn't work I was considering an adjustable gas block. But with a 12" DD lite rail (and pinned AAC muzzle brake) it would have been throwing out hundreds of dollars. So, I went over to heavybuffers.com and picked up their 8.25oz "XH" buffer. Last resort pretty much.

Took it to the range and it locked up on like the 5th shot. The rifle had an auto BCG in it, but I had a semi carrier floating around in my bag also. Never thought to use the semi carrier since it's lighter and didn't make sense that it would help to make that lighter. But, the spring and the crazy heavy buffer didn't help and the gun would not shoot reliably so I tried it just for kicks- fully assuming it would fail immediately.

A few hundred problem-free shots later, and a HUGE smile later I'm confident the problem is fixed. Thank you guys for all your help with the suggestions of springs, buffers, BCG's, etc. I'm oh so relieved this thing is working now!