Olympic Arms K10 This is an AR chambered in the destructive 10mm pistol round. Has very odd and cumbersome stick magazines that only hold 18+1 rounds. I think the design was made to fill up the entire magazine well. The other options are to use Grease Gun magazines which are long and thin but hold more rounds. 10mm will fit into .45 ACP magazines. Caution is required since they fit, you have to know what your magazine is loaded with. Shooting 10mm in a .45 ACP firearm can result in overpressure and severe injury or even death.