The Colt 9mm SMG is a closed bolt, blowback operated SMG, rather than the conventional direct impingementgas operation of the standard 5.56×45mm M16 type rifle. As a closed bolt weapon, the Colt SMG is inherently more accurate than open bolt weapons such as the Israeli UZI.
The overall aesthetics are identical to most M16 type rifles. Changes include a large plastic brass deflector mounted toward the rear of the ejection port. 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 magazine well of the receiver is modified using a special adaptor to allow the usage of smaller 9mm magazines. The magazines themselves are a copy of the Israeli UZI SMG magazine, modified to fit the Colt and lock the bolt back after the last shot.
Current Colt production models are the RO635 which features a Safe/Semi/Full Auto selective fire trigger group and the RO639 which features a Safe/Semi/3-round Burst selective fire trigger group. Both are equipped with a 10.5 inch length barrel. The 633 was a modified compact version with a 7 inches (180 mm) barrel and simplified front sight post used by the DEA.
The most common model is the 635, the latest version of which are simply marked SMG 9mm NATO.
^Technical Memorandum 2-79. AIMING POINT DISPLACEMENT FROM FIRING A RIFLE FROM THE OPEN-BOLT POSITION. by Dominick J. Giordano I. February 1979. U.S. Army Human Engineering Laboratory 1, U. S. ARMY HUMAN ENGINEERING LABORATORY, Aberdeen Proving Ground, Maryland 21005