First off i live in md and can not buy more then 10 round magazines with in the state but it is perfectly legal to own and use any size magazine in the state as long as you are not committing a crime with it.

My new Ruger max 9 came with 2 ten round magazines.  My first range session i used the two magazines that came with the gun and no issue. 100 rounds shot offhand. Last payday i went to the nearby out of state gun shop and  bought a twin pack of factory 12 round magazines.   I loaded them up at home and they locked tight like they are supposed to.

This morning was the second range session with the pistol. I took the brand new 12 round magazines with me.

I started the session with shooting off a rest simulating shooting over cover. This is where the issues started. While shooting the magazines would come unlocked and drop a little so the next round would not feed.  Both magazines did this several times in a row. I thought i may have been hitting the mag release by accident but i watched my hand placement and it was nowhere near the release. The second string of live fire was standings and shooting unsupported. I fired another 4 magazine full and they didn’t drop down.

Any idea why the mags would drop while the barrel was supposed on a rest but when shooting offhand?