Just bought a 16" upper for my Bushy.  Went out to Rio to sight it in.  The sight in went great.  I used the IBZ method.  I did however have about 4 or 5 jams during the first 100 rounds of 200+/- rounds fired. These were failure to eject type of malfunctions where the brass would get caught jammed against the upper part of the receiver/charging handle area when a new round was trying to get fed into the chamber from underneath.  So I would end up with an empty brass jammed on top of a new round coming out of the magazine trying to get into the chamber.   I disassembled and cleaned the bolt and bolt carrier.   I found the gaps in the 3 gas rings all lined up together.   I know I spaced them 120 degrees apart when I cleaned the upper after receiving it.  Did I get "short stroking" caused by the mis-alignment of the rings?   I know they have to be spaced, but I also thought that just because they aren't lined up the weapon should still function.  Correct??   Any ways, after I cleaned the bolt and carrier and the extractor, I re-aligned the rings, verified proper ejector function, and re-assembled everything,  I put another 125 rounds out of it without a problem.   The ammo I used was the South African manufactured stuff that I got through J&G Sales.  This ammo has cycled fine through my 20" upper with no problems in about 400 to 500 rounds downrange.  Now, can I attribute this to "breakin" issues or can this be attributed to "short-stroking" caused by mis-alignment of the gas rings??

Thanks in advance for any help given.

Your problem is the extractor is dropping the case once the spent case is out of the chamber, but before the carrier reaches the back of stroke. The problem could be caused by a rough chamber, weak extractor spring, or a burred extractor/bolt to name a few.

Check the extractor to make sure that it will move freely in the bolt channel. Then check the front of the bolt channel/extractor contact pad to make sure that this no burs that are holding the extractor partial open.  Then check the claws of the extractor and the end of groove in the extractor rim channel to make sure that the extractor fully locks/rides on the rim.

You could just add an O-ring (read D Fender) to the extractor spring to add tension to the spring or replace the spring to a stronger style.  This will allow the chamber to self-polish, and the burs to be knocked off the other parts. After 600 rounds, you can just remove the O rings/stronger spring, when the rifle self polishes out the problems.

That sounds like what I was seeing Dano523.  I just stripped my bolt and looked at the extractor and the extractor channel in the bolt itself. I did notice what appeared to be, under close magnification, a small "nub" or "ball" of material to one side of the extractor channel in the bolt.  This appeared no more then 10 or 15 mils (.010 to .015) in height.   The upper surface was polished where it appeared to be hitting/rubbing the extractor as the extractor levered back and forth in the channel.  There was a corresponding polished spot on the extractor itself.  I took a fine flat needle file and removed this nub and polished the inside area of the extractor channel lightly.  I also took the time to polish the extractor surfaces that rub in the channel with a fine Arkansas stone and some oil.  I checked the operation of the extractor in the channel after this and all felt/looked ok.   As far as the rough chamber goes, after I cleaned my upper after receiving it I took a round and dropped it in the chamber and then held the barrel assy straight up and down with the muzzle down and gave it a firm tap on the floor to seat the round.  I then inverted the barrel assy. and the round fell right out.  Other then this method, how can I tell if I have a rough chamber?   And if it's rough how does one polish it up?   I suppose if it gets to that point I should probably just send it back to BM.  I just hate to let it go.  I just got it.  

Thanks for your help.

Your check doesn't force the brass to flow into any rings or grooves in the chamber wall. Only firing a cartridge will do that. You can inspect the fired brass for any odd marks. They'll be lighter in color than the rest of the case. If the rings are deep enough you may even be able to feel the ridges on the case with your fingernail.

Best bet for checking the chamber is a reflector tool. Anything that can be mirror polished and will drop into the chamber and stop in the neck arear will work. I have several, some are made from firing pins with the rear spool ground off and the rear of the front spool polished and some are made from the rivets used on the sideplates of M1919 MMGs.



[Norman Stansfield]
"BEEEENGO!"
[/Norman Stansfield]