The Colt model 635 submachine gun was developed circa 1982, and currently is offered by Colt for military and law enforcement users. It was or is used by numerous police agencies in USA, as well as by US Dept. Of Energy Nuclear plants security units, US DEA, US Marines and some others.
Model 635 is based on proven design of M16A2 assault rifle, modified to fire less powerful pistol ammunition. Modifications are: barrel without gas port and gas pipe; simple blowback bolt; modified magazine port; CAR-15 - type retractable buttstock.
Model 635 submachine gun fires from closed bolt to improve accuracy and maintains M16-type trigger group. Model 635 has three-position safety-fire selector with available modes of fire of semi-auto and full-auto. Model 639 is similar to model 635 except that it features M16A2 type trigger, with semi auto and 3 round burst modes of fire. Model 634 is a carbine variant with only semiautomatic mode of fire available. Model 633 SMG features even shorter 7 inch (178 mm) barrel with simplified front sight post.
The main advantage of Model 635 SMG is that its controls and appearance is almost identical to M16 assault rifle, making it much easier to transfer users from one weapon to another.