I've got a strange thing going on with my .300 blackout pistol. The last round in the magazine always jams, ending up with the tip up higher than it should; missing the chamber opening and smacking into the area above the chamber. Basically looks like a short-stroking issue to my untrained eye. It will sometimes do it at other times as well, but it ALWAYS does it on the last round in the magazine. (I've blown through over 200 rounds in the last couple weekends dinking with it.) This is all with full-power ammunition; Fiocchi 150 fmj and Remington 130 Hog Hammer.
I've tried it with new L5AWM magazines, almost-new NHMTG/Okay, and almost-new Elander gen-3's. (I'm one of the few people who like Elander mags, apparently.) The gun has a CMMG 8.5" upper on a PSA lower, and the only modification I've made to it is a Superior Arms click-adjustable gas block, which is aligned well and Rocksetted in place.
When it happens, it looks like something that would be a result of short-stroking - the bolt is on top of the case instead of behind it. To vary & test bolt speed, I've tried the gas port nearly closed, wide open, and several settings in between (from 10 clicks open to more than 20), with no change in last-round jamming. The ejection pattern varies between 1 o'clock and 5 o'clock, depending on gas block setting. The bolt always locks open when empty, which seems to contradict short-stroking.
The gun hasn't had this problem before, but I've mostly run 110-grain VMAX and misc fmj stuff in the past. The reason for changing ammunition was because I'm about out of VMAX and have a 400-round can of the 130 Hog Hammer.
Any ideas would be greatly appreciated.