The M21 can incorporate picatinny rails for mounting accessories like optics, vertical grips, bipods, etc.

The Zastava M21 is gas operated, long stroke piston with a rotating bolt locking system rifle.

[3] It features a hard chromium-plated, cold forged standard rifled or an optional polygonal rifled barrel, integrated 22mm flash hider grenade launcher, heavy-duty synthetic furniture with a polymer folding stock similarly to the AK-74M, and side optics rail as standard (optional adapter base for cover mounted picatinny rail also available).

Barrels are also hard chrome plated to provide a longer service life.

A set of picatinny rails on the hand guard can mount various optoelectronic devices.