Sent off for a Bushy 16" flat top.  About 1000 rounds went thru it fine.  It is clean, and the ammo is good (SA).  

It either does a bolt over, leaving a scratch mark on the top cartridge in the magazine after ejecting successfully, or does a nose up jam with the bolt denting the middle of the cartridge, again after ejecting successfully.

Tried a variety of magazines.  Tried two different lowers.  Tried other uppers with the original lowers and magazines.  Upper jams the same with all (about one in five rounds).  Determined it is NOT the lower, ammo, or magazines.

Gas tube aligned fine.  Bolt cams in carrier fine.  Bolts from other uppers (did not attempt to fire with 'wrong' bolts!) worked the same in the upper when manually working (unloaded) action.  Firing pin moves fine.

What is left?  An obstruction in the gas tube?  Is this the type of jam that would occur if a bit of jacket had gotten shaved off and partially blocked the gas tube?  What other test should I try?  I am not looking forward to taking the handgaurds et al off.

FWIW:  The front site was replaced with a Bushmaster three rail block (BMAS).  

Thanks,
-Ngog

You say it's kicking out shells fine so the extractor & ejector should be ok. Might want to check em' though.

Did the problem start after you added the BMAS? If so, double check that. Make sure everything is tight and that the gas tube is clean. It sounds like you're losing pressure somewhere.

Check that the carrier key is tight and clean.
If not, clean it, replace the screws and torque to the proper rate. Loose carrier keys are a major culprit.

Check that the gas rings are in working order.

Check that the gaps in the gas rings are evenly spaced. Some say this matters, others disagree.

Take the buffer spring out and clean it just for the hell of it.

That's all I can think of at the moment.

Lets start from the beginning, The rifle is fully ejecting, and locking back on the last round.  By fully ejecting, you do mean that the case is being thrown a good 10' or better.  If this the not the case, the barrel gas passage may be blocked. Start with the gas tube flow(use a tube to test), then move back to the key to confirm that it is tight against the carrier.

The signs that your rifle is showing is that it is short stroking, and somewhere you have blockage, or a leak.

Pull the gas block and check the alignment or the two, both set back and rotation.  On one of my rigs, I had to keep the block off the shoulder a few mm to keep the ports aligned. It also helps to use a pencil line on the barrel shoulder(gas port index) to blind align the two.

Let use know after you have flow tested the system.  Use a piece of tubing if needed.

 I had the same problem with my carbine, and it turned out to be a slightly weak extractor spring.  The carbines have a tendency to open the bolt a little early in the firing cycle, and this is exacerbated by hot ammo, which ceertainly describes SA M193.  The case is withdrwan far enough to come loose from the chamber and is essentially blown back against the ejector by the higher than normal chamber pressure.  The fact that you have gone thru 1k rounds with no problems points to a wear issue, more so than incorrect assembly.  You may want to increase your mag catch tension by a turn or two.  Good luck.  Ops

Thanks for your replies!

I took things apart and the problem seems to be that the  Bushmaster three rail block crept forward about 1/32".  It uses three set screws instead of the normal front site/gas block's two pins.  

I moved it back into correct position, torqued them down, and went to the range.  Works like new.

Did a web search for gas blocks similar to the BMAS, but that use pins, and did not find any (Of course I am on a 26 Kbps, so I usually fall asleep before the search completes).  That would seem to be a much more reliable system.  I like being able to put my Glock's M3 light on the AR, and want to keep the accessory rails.  Maybe I'll fire up the drill press later...

Ngog_Nrythrng,

Sounds like you found the problem.

As for the drill press, you already have found the solution.
 Just counter sink/indent the barrel at the setscrews locations to allow the screws to hold the block without moving, and to keep the block from walking in the future.

Dano