First off, a disclaimer...I am the ultimate AR noob, but have been picking up bits and pieces from this great website!

This question is coming from a problem that arose in my new Bushmaster.  I had run approx. 100-150 rounds through it before I started to get the failure to eject, double feed.  (one live round, one spent casing) Patrolman's Carbine

Having read other posts with similar issues, I came across the troubleshooting method of "blow testing" the carrier key.  I have tried this and need some info.  I found a straw that fits perfectly snug on the carrier key.  With the bolt in the rearward position, I blow as hard as I can.  Am I supposed to have ZERO leakage (turn blue in the face) or is EXTREMELY MINOR hissing of air normal? (into the bolt carrier)  (this was all after checking the key for tightness and it is staked solidly)

My other question is, is it reasonable to assume that there is plenty of gas pressure (ie no serious leak) if the bolt reliably picks up the new shell?

I have never heard of a carrier blow test and can not imagine why someone would do one. All I look for is the key seated flat, the screws are tight, and no cracks in the metal of the key anywhere.
Sounds more like the extractor is slipping off the empty leaving it laying in the upper.

run USGI mags and military ammo

To do a leak test, you would need to make a plug that would seal off the gas port in the carrier gas section (read a plastic plug with two O rings, before and aft of the port).  Then using soapy water around the carrier/key and air compressor, charge the system.  Chances are you might get a few small bubbles from between the key/carrier (fouling will seal these is due time), but if you got big bubbles, you got a major leak that needs to be dealt with.

To solve a leak, you hand lap the key into the carrier using fine lapping compound, then when you go to reinstall the key with new allen screws, you use loctite (not as a bolt locker, but to make a thin gasket between the key to carrier surface to seal any defects in the surfaces that you may have missed during lapping).  

Also, before you start to lap anything, take steel counter sink and relieve the two carrier allen screw holes on the carrier (Read remove any high ridges around the openings before lapping or these ridges will just eat groves into the key matting surface), the gas port entrances on both the key/carrier, and confirm that the key/carrier ports do align together (read if they don't match, then enlarge the gas port channel opening of the key until they do (the carrier is hardened and the tougher to elongate).

To add, in regards to your Busy bolt, just put a allen wrench on the key screws and see if you can tighten the allen screws more (37 in lbs).  If they turn with hand pressure (no cheater bars), then the allen screws are loose, and new screws are in order to solve the problem (read the old units may be too far streached, and retightening them could just lead to broken screws that are just way too much fun to dig out of the carrer when it happens).

Thanks for the replys.  I had previously cranked hard on the key screws and they are SOLID.

Because I suspected the extractor spring, I grabbed the #60 O-ring and installed it.  I haven't had a chance to test fire yet, but hope to this weekend.  

I have since also learned to keep the chamber clean and free of oil to keep chamber pressure from spiking.

Looking at the extractor, it looks rough and the edges rounded.  The lip itself isn't as crisp as I would expect to really grab that shell and hold thru the full ejection.  My HK usp is pointed almost like a chisel.  Would it be adventagous for me to clean up the extractor a small bit with the dremel to create a clean 90 degree lip, or should I give the gun a test 1st?

Can anyone recommend some good tests for reliability?  ie....firing upside down, shooting one handed, etc.....  how many rounds should I shoot and not expect any problems???

Sorry for all the noob ????

The O ring should band aid the questionable extractor/spring.  If the rifle runs with the band aid, a new extractor and extractor spring may be in order to correctly solve the problem.

As for your pressure test, load a single round in the mag/fire and see if the bolt locks bock on the catch. If it does, even with leaving the spent round in the cahmber, you gas system should be fine (in regards to short stroking, but if a new extractor and spring or just the added O ring do not solve the dropped/left in the chamber spent case problem, you may have a bit of over function on you hands (read the gas system working a bit too good, or the gas port oversize ported).

Thanks for the reply.  I'll try the short stroke test, but the gun has never failed to pick up a new shell.  Always double fed 1 spent, 1 full cartridge and only periodically after the initial 100-150 rounds.

Also never left a shell in the chamber....the extractor seems to be letting it go early, before the ejector can pop it out of there.

After a second, more thorough cleaning, out of 80-100 rounds only 1 double feed.  My extractor felt a little spongy, so I'll try the o-ring, and if that does it, a Wolff HP spring will be ordered.

In my humble opinion I'm 99% sure this is extractor related.  Read at Bushmaster's site to check the ejector for proper tension and binding as this may contribute to my issue, but from my prior cleanings things felt good.

Finally, I'll try the pressure test, but the rifle has never failed to lock back on an empty mag.

Fired 100 rounds this weekend, slow fire, bench rested standing, rapid fire.....not 1 hiccup.

I cleaned up the area around the extractor claw with some sandpaper, so it would get good purchase on the case rim.  I also had used a #60 o-ring.  Locked back after every mag also.

Looks like it was a weak extractor spring.  A Wolff HP will be ordered soon!