Hey guys. I was hoping to pick the brains of some of the fine folks here. I’ve been having issues with my CMMG piston carbine failing to eject the spent casings anywhere from 1-2 times per mag or so.
What happens is the round is fired, extractor begins to remove spent casing, spent round gets stuck with the mouth sticking out of the chamber and casing is perpendicular to the rifle. The spent casing is stuck with the base caught on the left side of the chamber and if I recall correctly, stuck on a partially chambered round. The only way I can clear the malf, is to drop the mag, lock back the bolt, and pull the spent case to the rear of the chamber, loosening it from the live round partially loaded into the chamber.
I’ve tried getting an upgraded extractor spring and o-ring from BCM, but that only caused the rate of malfunction to rise to 6 or so times per mag. I’ve removed the o-ring and reinstalled the BCM upgraded spring, but I haven’t tested this yet. The gun is running a Spikes ST-T2 buffer, but I cannot remember the buffer spring. I do not think it is stock though.
If any of y’alls have any suggestions (short of “sell that POS and buy a __________”) I’d appreciate it! Thanks fellas!
is this a complete oem upper with pinned gas block or a strap on with cap screws?
if it is a strap on you can check gas block alignment yourself, screws may be loose.
gas port alignment is a critical on gp rifles as are the di systems.
gas system regulator is on or off. check for any obstruction in the gas ports.
check for any alignment issues on the op rod also. no binding or bent rod.
*also check your ejector. bad spring or crap in it. oiled? check with a spent case.
if it is the oem upper get on the horn with cmmg and see about an ra for repair.
My first guess is that you are experiencing some form of short stroking. You shouldn’t need the O-ring and if it is increasing the rate of malfunctions then it is because the extractor tension is too tight.
Have you checked the ejector to see if it is moving freely? Is the BCG new also or just the upper?
What kind of buffer are you using? If it is a carbine buffer, the face will be plain. Otherwise it will be marked H,H2,H3.
Had a similar issue on an 7.5" SBR at one point. It was way overgassed and the buffer was too light, causing it to cycle before the round could be ejected.
I’ve removed the o-ring to see if that will solve the problem. It still has the BCM upgraded extractor spring.
I have not checked the ejector. The whole rifle was purchased together. Nothing is really newer than anything else, except for upgraded buffer, extractor spring, handguards, compensator.
So this is a gas piston gun. Can you tell us what ammo you were using? Has anything been modified or changed on the gun or is it stock? What type of BCG is in the gun (M16 style vs AR-15). I understand it is a piston gun so the BCG will be slightly different. What magazines were used? Did some magazines have the issue more than others?
As IG said, short stroke sounds like the culprit. This sounds like a Type 2 malfunction, “stove pipe”, caused by your BCG not fully cycling/short stroking.
One thing that can cause this is a combination of weaker or under powered ammunittion, coupled with an increased amount of reciprocating weight (your BCG, Buffer and Buffer Spring tension). The ST-T2 weighs 4.3 ounces.
Let’s review buffer weights: (Thanks to Robb Jensen on another thread)
CAR buffers = 3.0oz
H buffers = 3.8oz
H2 buffers = 4.6oz
H3 buffers = 5.5oz
Rifle buffers = 5.2oz
You may want to try and test a lower buffer weight see if that resolves the issue. Or try a different buffer spring or both. Try that with the same ammo that caused the problem and see if the issue resolves. An H2 or H buffer with a Sprinco Blue spring might do the trick if that is the issue. There is a ton of information on buffer weights and recoils springs all over this site that you can read and search out.
To simplify it, if a weak ammo is having to throw more weight back than it is capable of doing in order to facilitate the extraction, ejection and feeding process, then you will have issues. Hope this helped.