My Saiga Krebs Custom KTR-03S in 7.62x39, had a donut bulge in the neck like that. A gunsmith at our range said that I might have a difficult time in reselling the gun. I explained to Mr. Krebs about the problem...and he told me it would not effect accuracy or ejection --- But I pressed on --- and he accepted to re-barrel the rifle for me.

I've seen one other empty 7.62x39 shell casing --- with the donut neck shape --- lying on the ground at our range; but I don't know from which rifle it came from...but I figured it was a Saiga.