Also if you get air coming out the back of the regulator it could be the reg seat. If replacing that doesnt fix your leak (may be accompanied by barrel farting) then your valve piston is probably bad.
Air whooshing down the barrel can also be caused by a bad sear. your sear should have a pointed edge where it catches the bolt. If the edge is rounded, then it is no good.
Just thought I would give some input on some other mag problems that I have experienced since they aren't usually talked about much.