OK, I finally had a chance to make it back out to the range. A couple of things have changed:
1. The more I've shot the gun the more I suspected that the magazine was at least partly to blame for my troubles. I decided to spring for a Lancer 10 round mag (a/k/a 4 round 450BM) and a Tromix follower. I used this magazine today.
2. Those 250gr FTX bullets are expensive!
I switched to 225gr FTX's for today's testing. Starline Brass, CCI magnum primers, 38gr of LIL GUN and a COAL of 2.13" or so. The powder charge and COAL were chosen because multiple people had good things to say about the accuracy of this load. Crimp was taper crimp with the same crimp I'd used on the 250's (0.473" I believe).
3. Today's testing used the stock buffer spring and the Spike's T-2 buffer. I've used this before but
So how did it go? Not perfectly but a lot better. I loaded as many as four rounds into the magazine at a time and fired a total of ten rounds. The gun cycled properly every time and there were no jams.
There were a couple of issues: Ejection still seemed pretty weak and somewhat inconsistent to me. Also, I shot the magazine empty three times and the bolt failed to lock back on the empty magazine in all three tries.
I know that an earlier post said that these uppers were typically overgassed but is it possible that something is causing it to be undergassed? I know that 38gr of LIL GUN is (according to Hodgdon) minimum charge for the 225gr FTX but it seems like lots of others have used this load without cycling issues.
As for the hammer and buffer spring: I've oiled the buffer tube and spring previously so I don't think they're the issue, and the trigger is an aftermarket all-in-one CMC trigger with a VERY smooth hammer face.