I've got one of the newer type 21s pistols and have shot over 400 rds. of xm193 with no problems.  Yesterday I had 3 bolt over bullet failure to feeds out of 150 rds.  Only thing I can think of is that the mag. was rested on a bench causing the problem.  Anyone else experience this?  What else could it be besides a mag. problem?  

you mean the bolt did not strip a round out of the mag?
if thats the case maybe your mag spring is to weak and is not pushing the rounds up fast enough.
all i can say is go out and try it again and bring diffrent mags.... if you think resting the mag on the table was the problem then try not doing that! try diffrent things.

There is an earlier post about a 97S pistol not feeding correctly or reliably. Check that post, it basically says send your gun or upper back to Bushmaster for work. You may have a feedramp that bushmaster will rework. They will likely offer to take a look at it and may end up fixing your problem. Mine had a feedramp that had channels that were too aggressive. Sent it back and they changed the feedramp. Have not yet fired it though.