Greetings,
"We", as in my co-workers and I had the same issue with short barrelled Bushmaster weapons that were issued to us. All but one of the guns jammed repeatedly.......as I recall, it wasn't ejecting the fired case and was trying to feed in a fresh round. Several brands of ammo, several diff magazines, etc etc. After 3 returns to Bushmaster, the great people there simply swapped them for different models.
The culprit? A couple things. A while back they had a run of short barrelled guns with the wrong size diameter gas tubes...too small to make the gun function. In addition, we received word that there may be a "personnel issue" which accounted for the number of returns.
IMHO, the people at Bushmaster are really cool, they will take care of you, call them and they'll have you return it and make it right.
As a side note, you ain't kidding about the accuracy! With ACOGs and Aimpoints on the little Bushy carbines, we are shooting such small groups that the fullsize and M4Colts and DPMS guns look stupid in comparison....its not even a contest.
Good luck!