Quoted: Quoted: Quoted: The correct buffer for the MK18 would be an H3! The H and H2 buffer are for standard M4 and M4 SOCOM HBARs respectively, Commandos get the H3. and that Armorer you spoke of needs to go drive a truck or paint the ship, if the weapon does not function, FIX IT! RMiller The H3 is purley a aftermarket product.   But its made by Colt for SAW.

OK, so the H3 is purely a aftermarket/custom part?

had the same problem when I went to a 11.5" barrel. Put in a H3 buffer and new hammer spring. No problems since.

Aftermarket part made by Colt???

Quoted: Aftermarket part made by Colt???

No, custom made by Colt for Ken.

Sorry I'm late to the game fella's but this is a classic case of bolt bounce. Easily cured by swapping out the existing buffer to a heavier one. Oh yeah, all of that has already been said.

Most folks blame timing or gas ports, but usually a problem like this is easily solved by using a slightly heavier buffer.

This is why you don't want to bore out your shorty gas port to .100 when you cut off the barrel. That's too much gas! It will slam the carrier to the rear of the buffer tube, the carrier will rebound, the spring will keep up with relaxation velocity, and then the carrier will smash into the locking collar. The hardness of the carrier and the locking collar will repel each other like water repels cats! (Try to throw one in a bathtub and see what happens!)

The resulting high velocity impact will cause the carrier to rebound with more velocity and inertia than the weights in the buffer can overcome. While all of this is going on the auto sear has been tripped and the hammer is well on its way to meet, not with the firing pin, but the carrier. All of the hammer's energy is spent on the lower ledge of the carrier when the bolt group comes to rest. The hammer will be down resting on the firing pin and there will be a round in the chamber with a dent in the primer that is somewhere between a chambered round and a fired round. Deeper than the dent from chambering but not deep enough to fire it.

I've seen it a LOT.