If you're not used to the HK mag release, you're probably right in thinking that you are inadvertantly hitting it with your trigger finger. Try holding your trigger finger a bit higher on the trigger and see if the problem persists. And the idea of using a THUMB to hit the mag release??? Who does that? Certainly not an HK shooter! I use my middle or trigger finger to hit the mag release -- whether I'm shooting righty or lefty!