If you're building or buying a 9mm AR-platform carbine (commonly called an "AR9"), one of the first and most important decisions you'll make isn't about the barrel, the trigger, or the optic — it's about magazine compatibility. Unlike a standard 5.56 AR-15, which uses a universal magazine well, AR9 lowers are split between two competing standards: Glock-pattern and Colt-pattern magazines. Choosing the wrong one — or building a lower that isn't compatible with the mags you already own …