I have only one MP5-type weapon, a DJF clone. It came with one of his mags, which works. The bolt rubs the feed lips a bit on that one. I just got some of the latest gen ATI mags, and they're literally unusable in my gun. The bolt is hitting the feed lips so bad that you can't even lock the mags in with the bolt closed (I know that you're not supposed to reload with the bolt closed, but I'm talking empty mags here). The ATI mags lock in TIGHT with the bolt open, then the bolt tries to occupy the same space as the feed lips on the mags, so I doubt these will even cycle. Is it the mags? I don't know because I don't have any real HK mags to try yet, but I have a couple on the way. So when they get here, I'll try them and then I'll know if I'm dealing with 100% crap mags, or possibly an out of spec gun.