I ran into something a few weeks ago I had never even heard of being an issue. I picked up a BCM lower finally and a few days later went to the range. To my amazement one of my mags didn't lock in. So I separated the mag that didn't lock in and went about my range trip. When I got home I tried a few different lowers, emptied it, and loosed the mag release. Still wouldn't lock in (on the BCM). So I contacted magpul. Apparently Gen 3 pmags have some issues with the over travel tab and have issues with some lowers. So Gen2 pmags for me only from now on. I went through all my mags and the gen 3 are now dedicated 300 blackout mags which I don't have any issues with in those lowers.