Changes include a large plastic brass deflector protruding from the rear quarter of the ejection port, and a correspondingly shorter dust cover.

Factory Colt 9mm SMGs are equipped with a 10.5 inch length barrel and have an M16 style upper receiver, which means they feature a fixed carry handle, no forward assist and A1 sights (with 50 and 100 meter settings).

The 633 was a modified compact version with a 7 inches (180 mm) barrel, hydraulic buffer and simplified front sight post used by the DEA and the Department of Energy.

The R0991 features Safe/Semi/Full Auto selective fire is constructed with Rail Integration System (RIS) picatinny rails on the flat-top receiver as well as around the barrel which allows the easy mounting of ancillary devices, has 10.5" barrel and equipped with a third generation composite buttstock; The R0992 has almost all the same features to the R0991, except the selective fire mode is Safe/Semi/3-round Burst only; The R6951 has almost all the same features of the R0991 and R0992, but doesn't have selective fire and has a 16.1" barrel instead of the 10.5" one.

A suppressed variant known as the "DEA model" exists that uses an integral Knights Armament Company made suppressor covered with an M16A2 handguard.