Re: I have made a terrible mistake.
? I have a Mini -14 and several older steel mags, 20 and 30, and just recently bought two of the 30 ProMag polys, NO PROBLEMS! Yes they are slighly loose, but so are the steel ones of long ago.
And no, you shouldn't hold on to them when firing the gun, they're mags, not handles.
I read the HK post here, I have an aluminum 30 rd one that I had to work on when I got it, seems the follower had burrs that caused jamming.
You are what you do, when it counts.