Welding on a flashhider for any gun can often irretrievably ruin a barrel. Even when done right it does damage to the barrel. I would skip it. Not saying thats the problem just a possibility.
Keyholing can also be caused by improper bullet weights for the rifling twist, improper diameter bullets (shooting .308 bullets out of a .311 bore in the case of 7.62x39), or improper speed. This issues can arise from poor quality out of spec ammo or someone new to handloading.
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." L. Neil Smith
The problem with being stupid is you cannot simply decide to stop doing dumb things...