I had a 10 round mag hanging out the bottom of my MCM A5, and it was almost too long for prone (I used a 9-13 Harris). It was definitely usable, but reloading could be a pain depending on the position you are in. If I rested the rifle on the ground with no rear bag it almost rested on the mag, but it did clear. That being said, having 10 rounds is nice if you have a long string to run through. Ultimately I just opted for the multiple 5 rounders, as they don't get in the way when I'm moving, and it's quick enough to reload with a fresh mag.