Maxwell Atchisson actually built 9mm AR’s based on the XM177E1, prior to Colt taking on the project, which would be given to Colt’s lead designer Henry Tatro in 1982. Notable American users of the system include (or have included) the Drug Enforcement Administration (DEA), Department of Energy (DOE, somewhat famously used at nuclear plants), US Diplomatic Security Service (DSS), US Marine Corps, and the US Marshals Service. Listed foreign users have included the Bahamas, Bangladesh, Malaysia, and Panama.
Early Experimental Models
Some early designs had no bolt-catch, fired from an open bolt, and used an M1911 type grip-safety. The grip safety disappeared when the open-bolt design was dropped, primarily due to safety concerns. The M16 upper receiver omitted the cam pin slot and gas tube hole, but was otherwise the same. All production models would use an Uzi type magazine, modified for the AR15 type magazine latch and AR15 type bolt-catch.
Model RO635, RO639 (1985-until recently)
introduced in 1985, these are the most used models, and are still maintained by some military forces.
I have not been able to confirm the designation by the US Marine Corps. The US Marine Corps adopted this model simply as the “Colt SMG”, likely the largest procurement of any Colt 9mm model. The standard contour barrel is 10.5″ with a 1:10 twist rate* and utilized an A2 birdcage flash hider.. Both models have a 3-pos trigger with either full-auto (635) or three round burst (639). The trigger and hammer pins were upgraded to nickel plated stainless steel (sometimes seen on some earlier models), which lasted much longer with the SMG’s high rate of fire, a hot 800 to 1000 rounds per minute. Heavier buffers were needed to tame the cyclic rate below 1,000 rpm, a standard 5.56mm buffer would result in 1,250 rpm. Before going into production the large brass deflector was added, however not to deflect brass, but rather to deflect the extra gases the blowback design creates. Current 635 production models are simply marked “SMG 9mm NATO”.
The DEA purchased 633 and 635 models, as well as a modified 635 with an integral suppressor made by Knights Armament Corporation. This KAC suppressor model had a front sight post at the regular 635 sight-radius but had a modified M16A2 handguard that shrouded the suppressor several inches past the sight post. (I have not found a photo of this which is confirmed to be in the public domain.)
The model 634 is the same as the 10.5″ barreled 635 minus the full-auto position on the selector, making it a semi-automatic law enforcement short barreled rifle (SBR).
Experimental Model 633
The following is an experimental model of the 633 which lacks the front flip sight and has an aluminum handguard. More interestingly is the ambidextrous selector and bolt catch, trends that have been pushed heavily in the modern civilian market.
Model RO633, RO633HB (discontinued)
This model was known as the “briefcase” SMG, due to it’s short overall length, and more often as the “DOE model”. It is the seven inch variant of the RO635 model. The 633 trigger is 3-position full-auto and has a cyclic rate of 700 to 900 rounds per minute. The front sight flips down, the handguard is smooth and specific to the seven inch 9x19mm models, the standard contour barrel is 7″ with a 1:10 twist rate. Although most SMG models utilized the A1 upper, the rear sight was modified to 50m as the standard aperture. Because the SMG fires from a closed-bolt, a spring was added to the firing pin to keep it from striking the primer when cambering a cartridge. A weight was also added, pinned into the bolt carrier, to delay the extraction until cartridge pressures were depleted enough to safely extract. An alternate model was offered as the 633HB with a hydraulic buffer, though the standard 9mm model also used a heavier buffer than the 5.56mm models. Both variants are listed to weigh in at 5.4 lbs. Note the roll pin locations, holding the magazine-block in the magwell, vs the location on the other models.
Model ARO991, ARO992 (present)
At the time of this writing, the 991 and 992 are the current Colt Defense select-fire SMG offerings. Both models have a 3-position trigger with either full-auto (991) or three round burst (992). Both models have an A3 type “flat-rail upper” and quad-rail handguard. Listed at 6.7lbs with a 1:10 RH twist 10.5″ barrel (erroneously listed as a 1:7 twist barrel on Colt’s military website).
Model R6450 (1985-1988/89)
The AR15 9mm Carbine used a different semi-auto hammer, than its 635 full-auto counterpart, and was introduced alongside the 635 in 1985 (there is a conflicting report this wasn’t sold to civilians until 1987). The first commercial production model kept the semi-automatic A1 upper and A1 lower, initially without the 9mm brass deflector, which was added later, nor the M16 forward-assist. The magazine adapter was of the early two-piece design. Later production saw the replacement of the lower receiver to the A2 design and the addition of the large 9mm brass deflector. Total weight is listed at 6.3 lbs. The 6450 had an A1 flash hider, a 1:10 twist 16″ barrel for commercial civilian sales and was quickly replaced by the 6430 model. A model 6540 is sometimes listed in literature but appears to just be a typo, swapping the 5 and 4 digits. The model was temporarily discontinued in the late 1980’s due to political pressure against civilian sales of such weapons; the model reappeared as the AR6450.
Model R6430 (1992-1994)
The Colt Sporter Lightweight replaced the 6450 in 1992. A fixed stock replaced the collapsing CAR15 “third generation” stock, the larger .170 hammer and trigger pins were introduced, an automatic sear block was pinned in, as well as the removal of the bayonet lug. The A1 flash hider remained.
Model MT6430 (1994-2004?)
In 1994 the flash suppressor was removed, due to the passage of the 1994 Assault Weapon Ban (AWB), and the carbine was re-designated the Match Lightweight MT6430. Both versions used a 1:10 16″ barrel and are listed at 7.1 lbs.
Model AR6450 (present)
The return of the 6450 used an M4 type “fourth generation” collapsing stock until it was replaced with the “fifth generation” Super-Stoc and an A1 style flash hider. The handguard has also been replaced with the wider M4 type ribbed cylinder handguards. This 16″ model is currently available from Colt’s Manufacturing Company to civilians but was originally intended for “Law Enforcement Only” sales, prior to the 2004 sunset of the AWB. Current 6450 models are listed at 6.3 lbs.
Model AR6951 (present)
The 6951 is a flat-top A3 upper version of the 6450. Like the 6450, early models were shipped with an M4 type stock, but the current production models are shipped with the Super-Stoc. From the two models I’ve checked over, the rear sights were standard for 5.56mm. (Update: For 2014 Colt added a limited AR6951MPMG with Magpul MOE furniture hydro-dipped in a pink and purple camo).
It is unclear what the AR6950 is. On the Colt LE site it is listed as the AR6950 and AR6951 on the Features/Models tab but as the AR6450 and AR6451 on the Technical Specifications tab. It appears, what they mean, is the 6450 is available for LE as well as the 6951. I can find no other catalog or distributor reference to either the 6451 nor the 6950. On that note, Black Rifle II references a model 663 while showing a 633 and 633 prototype, this is likely just a typo and not an actual prototype 663 model.
* Bigger Hammer lists this as available in the 7″ (with front flip sights) or the 10.5″ barrel, but all literature lists the 7″ and 10.5″ models as separate catalog numbers and not options within an existing model.
References: Bartocci, Christopher R. Black Rifle II: The M16 Into the 21st Century (Deluxe First Edition). Collector Grade Pubns, 2004. P.126 Bartocci, Christopher R. The Colt 9mm NATO SMG/Carbine. Small Arms Review, 2008-08. Bigger Hammer Colt Commercial Variants. Last updated 1999-09-24 Colt Defense: Law Enforcement. Colt 9mm Submachine Gun (SMG). Collected 2013-04-18 Colt Defense: Military. Colt 9mm Submachine Gun (SMG). Collected 2013-04-18 Colts Manufacturing Colt Carbines. Collected 2013-04-18 Ganderp, Terry J. Jane's Infantry Weapons 1997-98 (Twenty-third Edition). Biddles Ltd, Guildford and King's Lynn, 1997. P.117-119 Long, Duncan. The Complete AR-15/M16 Sourcebook. Paladin Press, 1992 P.66-68 Muon, Jean. The M16 (English Edition). Casemate, 2004. p.81-84