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...
"I crapped my pants to avoid the draft!!" -Ted Nugent