Re: AR 15 7.62x39 range report...
First off, some of it may be a magazne issue.
The Pmag was never designed for the x39 round. True that you may can fit a few rounds in a mag, but using it would be iffy at best.
AR stoner mags historically have been made from various manufacturers, but in the case of x39 mags, there are only a few companies who have ever made them. Cproducts, and whatever the new company that is using their tooling is named. ASC or something like that????
Usually a short stroke problem is a gassing issue. Whether the front sight post is aligned with the gas port or a host of other things. A lighter buffer may do it, but I would wonder why the x39 round wasn't strong enough to cycle the bolt fully to begin with. The ST2 buffer isn't much heavier than a regular buffer anyway.