Hey guys, sorry for so many questions. I have a Ruger SR556. It came with 3 pmags with M4 stamped on the side. I bought 10 extra but they have M3 on the side. When I fire the last round from the ones labeled M3 the bolt doesn't catch. If I pull the charging handle back with an empty mag the bolt catches until I release the magazine then it closes. It works normally with the one labeled M4. I've fired less than 100 rounds through the gun it's brand new. Just looking at the mags I can't see a difference.