Wont lock back on empty magazine with overgassed upper

[MistWolf]

Hmm. A carbine buffer is right for well gassed guns or undergassed guns.

No. It's too light. Some ARs thought to be over gassed are actually under buffered.

Im no expert, but what weight buffers do the military use on their M4s?

Military issue M4s have H2 buffers.

First, rid yourself of the notion that ejection angle is any kind of indicator as to how your upper is gassed. The change in angle using higher pressure ammo is more an indicator your extractor spring is weak. Test the problematic upper with just the BGC from the properly gassed upper and see what ejection angle you get.

Second, not locking back is an indicator that your gas system is leaking and is beginning to short stroke. Magpul has made several rolling changes to their mags over the years. Older mags may require the carrier to travel rearward more to lock back. (Lock back point also varies between different brands of mags.) Inspect for leaks, worn gas rings and worn gas tube.

Third, not locking back can also indicate the mag release is improperly adjusted. The button may need to be turned in a bit.

Fourth, how do you know your upper is over-gassed? Ejection angle isn't a reliable indicator. (If you don't believe me, get a barrel with an overly large gas port and an adjustable gas block and use ejection angle to tune it.)

Fifth, if the buffer is a carbine weight, you're wasting your time. Replace it with an H, or better yet, H2 buffer.

Sixth, replace the extractor spring with a Colt or Sprinco M4 spring. Trouble shooting with a weak extractor spring is a waste of time. Extractor springs are cheap.

Seventh, check the length of the action spring. If it's too short, it needs to be replaced. If it's been used a lot, it could have collapsed.