K, my dad and I had enough extra and old parts laying around to put a little project together. He got a Brand New CMMG Piston Retrofit kit from Brownells. I had the A3 upper, and Bushmaster 16in 1/9 barrel from an older carbine.
The Piston kit came in, we followed the instructions, it looked right, but we are getting failure to extract and failure to feed. Plus REAL bad carrier tilt(I think), as its scraping along the at the bottom of the “entrance” to the buffer tube.
If we shoot XM193, we get 5-6 shots that shoot fine, then a failure to feed and even with the FA its still a bitch to get it to feed. Then everyone once and a while it wont extract. If we shoot American Eagle 55gr .223(only thing other than XM193 we’ve shot) it will only fire 1 shot then failure to feed.
The gun will cycle back far enough to extract for 5-8 rounds but not feed a new round in, THEN after 5-8 rounds we start getting failure to extract.
This is the first I have ever dealt with a Piston retrofit kit, same with my father, and we are baffled.
Im wondering maybe the Bushmaster barrel is part of the problem, being under gassed if there is such a thing. But I really dont know. I have a DD 1/7 M4 5.56 barrel that we can try, but would really prefer not to deal with it.
The Lower is a M&P15 lower, the A3 was a stripped D.S. Arms(bought from brownells). Maybe the FA is causing problems? Did I not install that correctly. We really have no idea.
Piston guns are known for carrier tilt. This shouldn’t be a surprise. How old is the Bushamster? I am going to stab in the dark and say that you also have a tight chamber.
I believe that Bushamsters tend to be overgassed. You may consider using a heavier buffer (H1 or H2) but since I have no experience with the CMMG kits I can’t say for sure.
I would try and find someone with a chamber reamer and ream the chamber. Ultimately I would remove the kit and send it back to CMMG. YMMV.
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.
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.
I happened to remember I still had the original Bolt that came with that Bushmaster barrel. Swapped out the one that came with the CMMG Carrier and the gun runs now. Something must have been off with the Bolt that came with the CMMG BCG.
I would still pursue the issue. This is the only way to hold companies accountable and hopefully force them to have better QA/QC and make a better product.
