A buddy of mine just finished building a 7.5" SBR. We took it to the range to test fire it. I fired 1 round and the bolt locked to the rear and the case was lying on the top of the magazine. I switched the H buffer out to a standard CAR buffer and it did the same but had a different "feel" so i loaded 2 rounds in the Pmag and it cycled the first one and the second one was lying on top of the magazine like the first time. I then loaded 5 rounds and fired one round and the second one up entered the feed ramp and got jammed in with the spent case half way out of the chamber. I then took my bolt and carrier ( Colt semi auto with a cut away rear) out of my 11.5" and put it in his gun. The gun ran flawlessly. I put the H buffer back in and we continued to shoot it over 200 rounds without ANY problems(mostly rapid fire). We used several types of ammunition including 55 gr Hornady TAP, 55 gr Federal XM 193 Tactical, 55 gr Remington green box, 62 Gr Federal American Eagle, 68 gr Hornady BTHP and some reloads. No malfunctions what so ever.
Here's the issue... We lubricated the original bolt and carrier and put it back in the 7.5" gun. It failed to function the same way again. I then placed this brand new bolt and carrier in my 11.5" gun and it ran a 30 round Pmag without any malfunctions. The new bolt is a Bushmaster carrier (that needs to be staked) and a bolt from MA Parts. It had an upgraded extractor (black) but it did not have MP stamped on it. It is obvious that my buddy needs to purchase a new bolt but why did the one work in one gun and not at all in the other? My bolt carrier has less mass at the rear because it is cut. I recommended a BCM (not in stock) bolt or a DD (in stock) but if it is bolt weight, is this the right choice?

