The Colt Model 639 was the export version of the Colt Model 629, which was type classified by the US military as the XM177E2 and issued to MACVSOG special operations units in 1967 and 1968. Improved from the Model 609 carbine, the 629/639 has an 11.5 inch barrel and an interesting small muzzle device (“moderator”) which served to change the signature of its firing to sound much more like an AK type rifle than an M16. The device does that job well - at least until it had been fired extensively, which slowly fills up the (non-disassemblable) unit with carbon and powder residue, substantially reducing its effectiveness. It has a full-fence lower, standard carbine buffer and spring, and a two position aluminum collapsing stock. It is the iconic weapon of US special forces in Vietnam.

Only about 100 of the Model 639 were made in the early 1970s, and many of those were sent back to Colt in 1975 under a recall. At that time, the ATF decided to classify the muzzle device as a silencer, prompting Colt to recall the guns and remove and destroy the devices. Some owners, however, kept their carbines and instead registered the muzzle devices, allowing them legally remain on the guns.