Maxim Defense MXM48172: When youre looking for Maxim Defense ingenuity in a new standard AR-9, the MD-9 is your answer. The MD-9 was engineered to fit the growing demand for cutting-edge AR9 style rifles for todays Military and commercial market. For increased comfort and versatility every MDX features our free-floating barrel with our custom handguard. These handguards provide a monolithic-like upper receiver platform and M-Slot compatibility.