AA must have a thin block or something in the front of the magazine that limits the .223 rounds at 10, but the slightly shorter 50B rounds go past it in order to get 7.

That makes since then. Thanks. I thought they were just blocking the 20 round mag to hold 10, like Bushmaster does with its current 10 round mags. They are the same size as the 20, but only hold 10.