The Zastava M21 is gas operated, long stroke piston with a rotating bolt locking system rifle. It features a hard chromium-plated, cold forged standard rifled or an optional polygonal rifled barrel, integrated 22mm muzzle device grenade launcher, heavy-duty dark/black synthetic furniture similarly to the AK-74M with a polymer side-folding stock, and side optics rail as standard (optional adapter base for cover mounted picatinny rail also available). The rifle can also mount a 40mm underbarrel grenade launcher. It is fed with standard 30-round magazine. The cyclic rate of fire is 680 rounds per minute, sustained one is 120 RPM.
- M21 A – Standard baseline assault rifle.
- M21 ABS – Built-in picatinny rail system.
- M21 S – Compact short barrel assault rifle.
- M21 SBS – Built-in picatinny rail system.
- M21 C – Carbine.
- M21 BS – Built-in picatinny rail system.