The magazine allows you to pull the collar down as you drop the ammunition into the magazine. This makes it possible to drop several rounds in at a time and load magazines in a fraction of the time with far less effort. The Speedmag works in all Mil-Spec AR-15 lower receivers and will accept any ammunition that fits in regular mil-spec magazines like .223 Remington, 5.56 NATO, and .300 AAC Blackout.