CMMG gas piston conversion short stroking, ftf, fte.

[trbii]

I’ve got a 15-20 year old Bushmaster XM-15 E2S 16” barrel, carry handle, iron sighted, old school carbine with this piston conversion installed, that is failing to function. Bolt doesn’t travel back far enough to strip the next round off the magazine. Have to manually cycle the action for every shot. A single shot repeating rifle. Very disappointed, clueless. Figured out the gas block was loose, Torqued it back down, took it back out to the range, fired a few rounds, 20-25 maybe, normal function, then same fte, ftf. Dead trigger or dry snap? As if minimal amount a gas was making it thru the barrel port. How do AR mechanics know exactly where the barrel gas port hole, is?

 

[Firemedic712]

Possibly a buffer weight issue?

 

[trbii]

Possibly a buffer weight issue?

Tried to research that, my Bushmaster does have a rifle length buffer/spring in the RE. Showed it to Darren, over at Action Arms gun shop. He said it was the correct unit for my platform.? Damned if I know. Before this, I identified carbine length/classification by barrel lengths, (under 18”, Carbine. 18” barrel or longer, Rifle) I’m not real up on gas system lengths, 7” short, 9” mid length, 11” rifle? Which buffer spring is correct for what?

 

[Firemedic712]

I'm running a h2 buffer and wolf carbine spring

 

[mr ed]

Are you using std round handguardswith the metal piece in front between handguard and front post?

 

[Fyrtwuck]

I had the same issue with an Adams Arms piston conversion. You need to enlarge the gas hole in the barrel to allow more gas to push the piston to the rear.

 

[trbii]

Are you using std round handguardswith the metal piece in front between handguard and front post?

Mid West Quad Rails, 7”

 

[trbii]

I'm running a h2 buffer and wolf carbine spring

Thanks for your response. I have been very tempted to switch buffer/spring, something I could do myself. The more I learn about options, more choices that exist, makes me hesitant to throw money at it. Trying to learn from those more informed on AR platform details. Owned around a half dozen different AR platforms over the last 30 years, competed with 3 of them, never experienced any function issues like this.

 

[Shinneryfarmer]

My son did a gas piston and we had to enlarge gas port to get it to function correctly. I had another upper with a enlarged gas port we swapped the gas piston to, functioned flawlessly. Didn't wont me drilling on his new barrel unless he new for sure that was the issue. Enlarged gas port on his upper and reassembled. Worked like it was suppose to.

 

[JEVapa]

This is a dead horse in the AR world but it's always fun to read and respond

Did it run before you put the kit on? Any Issues?

If it's short stroking and FTE, why would you consider a heavier buffer? No need for that mess. Focus on the issue at hand, which is the thing you put on the barrel of the perfectly good gun that is not working at the moment.

Also, those kits from CMMG are supposed to have an adjustment on them for regular and suppressed. If it's in the the lowest (more closed) position, it can malfunction...you're supposed to be able to almost close the port.


The two options I would entertain:
1. Remove the kit and return it to it's former self that you know works. Either sell the kit to some sucker on here or chunk it. Be happy with gun. I'm serious, not a joke.

2. If set on keeping it, you have to deal with its problems and know there's a good chance it's a terminally unreliable gun.
Reminder Per @mr ed use the shim/spacer that came with it or and least get it spaced off the shoulder correctly on the seat.

DISCLAIMER: I don't have any piston kits on any guns anymore. Not worth the hassle. All are direct impingement. Only piston uppers worth a crap IMO are purpose built from the company.

With that DISCLAIMER, these are things I have done to get a piston conversion set up correctly.

On the barrel, Mark centerline (of barrel-TDC) front and rear of the gas seat with a sharpie or something else you can see that doesn't jack your barrel.

On the barrel measure distance from shoulder to rear of port - write it down.

On the gas block, measure distance from rear of block to rear of port - write it down
do the math for the space you will need to offset the shoulder.

Measure both the barrel port and the gas block port (mostly so you know how much slop you do or don't have. They should be sitting right about .070 for a 16" carbine gas. Older guns, the barrel port may be as low as .062 or something. Whatever it is, the gas block should be at least the size of your barrel port. They are almost always way oversized for slop and for whatever their little adjustment system is.

Seat your GB per your measurements above. Torque your setscrews and use BLUE threadlocker. You can use green but life will be difficult if you need to remove it.
DON"T USE RED THREADLOCKER. You will need a torch to get it off and it usually requires drill bits and easyouts. Bad move.

Finish setup per the instructions.

When you shoot, make sure you are on the most OPEN setting. As you shoot a round at a time, dial it down until you have a malfunction. then back a couple clicks.
If there are only two settings, put it on the highest.
If there are only three or four settings, put it on the one closest to the middle, and turn it up if it starts to malfunction.

NOTE: If your gun ran fine before you slapped that Frankenstein neckbolt on it, there is absolutely no reason to spend time or money on anything else like buffers, trigger springs, bolt knickknacks, or whatever widgetry someone thinks will solve your problem. Your problem is the gas system you manipulated so that should be your focus.

If you have to drill out your gas port larger than what's on this list or what ran great for 20 years, you might be a redneck. Recommend you don't.
Now, I do see a lot of barrels with a carbine gas and a .075ish port...they do that so there's no doubt it'll run...not very good AR smithing but effective - P for Plenty I guess.
https://tacticalmachining.com/learn/ar-style-rifles/ar-15-gas-port-sizes.html