File your mags, not your firearm.
My WASR is a tight fit on some mags, I usually take a bit off the spine just above the rear locking lug if it hits the crossbar before it locks in.
Easiest way to check is remove the bolt & piston, then try to lock a mag in place and see where the interference is.
Normally you should get an audible click when the mag release snaps into place.
Filing AK mags is normal. Or just buy some PMAGs cause they are one of the few mags that seem to fit everything unmodified.