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.

The buffer weight may be off.
Try the lightest and heaviest you can find.

The gas system should be open when you try it.

If it's improving with a different weight.
Tune the gas system to look for further improvement.

Try an H2 buffer.

How do the feed ramps look?

Do all your magazines feed ammo from the same side on the last round? I've seen feed ramps be problematic on just one side.
Check the profile between the feed ramps and the receiver. Post pics if you can.

Standard spring I assume?

I would try a heavier buffer.

Fwiw, looks like everyone here was right. I only had a little time to mess with it after mowing a field last night, but I pulled an H2 buffer out of another gun and tried it. Only ten rounds, but zero malfunctions this time. I'll get an H2 ordered for it today.