How to Build Your Own Colt Model 607 Carbine

The DIY guide for building one of the rarest M16 variants ever

April 24, 2025 By Will Dabbs, MD

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When I was a kid, the world was awash in war toys. There was no orange paint on the muzzles, and many of them actually shot stuff. They could often pass for the real steel in dim light. Apparently, there were not quite so many plaintiff’s attorneys back then.

TV had about three channels when the weather was nice, and the accepted treatment for ADHD was exercise. As such, most every afternoon after school, the neighborhood hooligans would congregate in the nearby vacant lot, divide into two teams, and proceed to shoot the bejeebers out of each other with toy guns. Dirt clods pulled double duty as hand grenades, and a grand time was had by all. When one team would secure a decisive victory we’d generally mix everybody up, swap out weapons to keep things spicy, and go at it again until our moms stepped out on their respective back porches to call us home for dinner. Repeat as necessary…

My personal weapon of choice was a plastic M16 appropriately scaled for little hands made by a company called Marx. Pulling the trigger produced a satisfying burst of faux automatic weapons fire. Miraculously, I still have that thing.

The weapon in question bore all the hallmarks of Gene Stoner’s epic assault rifle. However, it looked like somebody had crushed it front to back in a vise. I always assumed this was just a byproduct of having had the real weapon transformed into a child’s plaything. Then I
matured into a serious gun nerd and came to appreciate that it was actually something way more elegant.

Origin Story

The Colt Model 607 was indeed a most curious little Frankengun. (Photo Provided by Author)

Disclaimer: The historical details that follow are fairly arcane and tedious by any reasonable metric. If you are like me (and if so, you have my pity), you should find it thoroughly captivating. By contrast, if you are a normal person with friends, pets, and sensible hobbies (like golf) you might want to skip a bit. That won’t hurt my feelings.

Developing the rifle that led to the M16, and subsequently the M4 carbine, was a collaborative effort. Starting out with the 7.62x51mm AR-10 battle rifle as a foundation, Gene Stoner and Company shrunk everything down to accommodate the smaller, faster, lighter .223 cartridge, itself also a Stoner design. At the time, Stoner and his team were working for a very small subsidiary of the
Fairchild Aircraft Corporation called ArmaLite.

ArmaLite was not configured for mass production. They were essentially a brain trust. As a result, ArmaLite farmed out actual production of these zippy little rifles to Colt. Colt took the design and ran with it.

Colt Model 601,602,603 — From top to bottom we have the Colt 601, 602, and 603. The Model 603 was a close approximation to the familiar M16A1. (Photo Provided by Author)

Colt engineers hoped to develop the M16 rifle into a system of weapons that could be sold to the military and make everybody filthy rich. They envisioned standard rifle versions, a heavy-barrel support weapon, a belt-fed light machinegun, and a stubby carbine variant. The design evolved along a fairly discrete path.

The original ArmaLite AR-15 fed from a straight 25-round box magazine and sported a trigger-looking charging handle located within the carrying handle. The subsequent Model 601 featured a chromed bolt carrier, an early duckbill open-tipped flash suppressor, and a slick-sided lower receiver with a removable front pin. It also came with a “waffle” magazine. The 602 was a similar weapon with a slightly modified (for strength) “forked” or three-pronged flash suppressor and a captive front pivot take-down pin. There was no fencing around the magazine release.

The Model 603 had the same general morphology as the familiar M16A1 to include the forward assist upper and full fence around the magazine release on the lower receiver. (A pre-603 transition model, designated as the XM16E1, had only a partial fence, and still retained a chrome bolt carrier of the 602 except that it had “teeth” for the added forward assist.) All of these weapons featured skinny 20-inch barrels with a 1:12 twist save the very earliest examples (prototype and 601 models) which were 1:14.

Four images of features — (bottom right) Palmetto State Armory offers some of the most delightfully exotic retro lower receivers. They actually sell one that is specifically marked to represent a late-model 607. (top right) The standard open-tipped three-prong flash suppressor was brutal in action. The stubby little moderator helped mitigate the noise somewhat. (Photo Provided by Author)

The Army still needed something more compact than the full-sized M16 for dog handlers, aviators, Special Forces, and the like. In 1965, Colt engineers went to work designing a carbine version of their revolutionary service rifle. The resulting Model 607 was actually described in their marketing materials as a submachine gun.

Convention dictates that a true submachine gun actually fire a pistol cartridge. However, we can forgive the Colt guys for their exuberance. The Colt 607 carbine actually paved the way for the entire family of M4 carbines so commonly encountered today. This quirky little rifle was really little more than a prototype.

Most illuminated gun historians agree that there were only about fifty 607’s ever produced. Serial number assessments put that number closer to 400. However, out of an overall production run of around eight million M16’s, the 607 nonetheless represented a drop in the ocean. Considering this handful of guns was essentially bodged together from bits and parts available at Colt, there was little standardization.

Colt 607 disassembled — Building a 607 clone is fairly easy. Accumulating all the parts you need is a bit harder. (Photo Provided by Author)

For starters, Colt engineers cut the standard 20-inch barrel in half. To accommodate the shorter top end, they took standard triangular M16 handguards and chopped them down as well. Some of the forearms were cut from the front, and others from the rear. Those cut at the rear used a custom tail cap to interface with the standard slip ring. Those cut at the front required a larger nose cap.

Lower receivers came in two broad flavors. Some were built on Model 601 lowers that had a removable forward push take-down pin and no fencing around the magazine release. These were typically either crudely marked with nothing but a serial number or were designated 607. Later versions featured the retained forward pin without fencing. These were typically marked GX-5857. The 607’s usually had a slick-sided Model 601 upper, while the GX-5857’s included the forward assist upper of the M16A1.

Early guns included the stubby three-prong tapered flash suppressor of the Model 602 and XM16E1. However, the muzzle blast from the abbreviated ten-inch tube was simply mind-boggling. As a result, Colt engineers developed a 3.5-inch moderator that helped tame the chaos to a degree. These moderators were not silencers in any sense of the word, but the BATF nonetheless still required NFA registration.

Three images of Colt 607 features — (bottom left) These things are tough to find. I tracked this clone 607 buttstock down on GunBroker but had to pay a premium for it. The brake securing the stock in place was built into the buttplate. (bottom right) The barrel on the Colt 607 was half as long as that of the standard infantry rifle. (Photo Provided by Author)

The pistol grips were standard M16A1 fare. They cut the grips on early versions down even with the bottom of a standard 20-round magazine before sanding them flat. However, the most striking aspect of the 607 was its buttstock.

These stocks were crafted from existing Model 602 furniture mounted on a proprietary collapsible skeleton. Mounting those originals required drilling and pinning the lower receiver. They included a lever mechanism in the back that acted as a brake to secure the stock either opened or closed. When extended, the stock length mimicked that of the standard M16 rifle. The original locking levers were not marked. As such, it was easy to extend the stock, fail to lock it in place, and then have it collapse on the first round fired. The end result was nonetheless compact and handy.

A handful of these 607 carbines were shipped to Vietnam for operational testing with Army Special Forces and Navy SEALs. While the compact size of the weapons was appreciated, the buttstock was both complicated and heavy. The handguards were also not terribly robust. Photographic evidence shows one Model 607 brought back from Vietnam with battle damage from a Combloc 7.62x39mm round that struck the left handguard. Feedback from those early trials led to the subsequent XM177 family of carbines. All of these cut-down weapons had their bayonet lugs ground away.

Where Do I Get One?

Colt 607 with helmet and grenade — By all accounts, the Colt 607 was well-received by the troops who used it. (Photo Provided by Author)

Building up a period-correct Model 607 is an undeniably arduous undertaking. The physical build is no more onerous than might be the case with any M4 parts gun. However, sourcing the specific parts takes diligence and a little cash. We started with Luth-AR for the basics.

Randy Luth is one of the nicest guys in the gun industry. Randy was the first guy to graft a rail onto an AR-15. If you have ever taken an AR to the field hunting you have Randy to thank for that as well. Randy Luth is the most influential small arms innovator you have likely never heard of. His company, Luth-AR, offers a wide array of top-flight parts for both modern black rifle variations as well as retro guns.

Luth-AR offered the stubby triangular handguards, the internal parts to include the bolt carrier, the slab-sided upper receiver, the three-prong flash suppressor, and the sundry sights and barrel components. I carefully cut the bayonet lug off using a cutoff wheel on a table saw and then touched up the front sight base with a Dremel tool. A little cold blue left this component looking factory new.

Colt 607 cut down grip — Most, but not all, of the pistol grips on the 607 carbines were cut down at roughly the end of a 20-round magazine. (Photo Provided by Author)

I sourced the ten-inch barrel from a vendor online along with a fake 3.5-inch moderator for comparison purposes. (NOTE: Phoenix Weaponry has NFA reproductions.) Palmetto State Armory (PSA) produces slick-sided lower receivers with the correct GX-5857 markings. Depending upon the specific model you wish to recreate, they sell lowers with both captive and removeable front pivot pins. PSA has become the go-to guys for exotic retro AR parts. Their wide array of vintage lower receivers is simply intoxicating. I have built dozens of guns from PSA parts and found them to be of great quality at a reasonable price.

It doesn’t take a great deal of talent to build this gun, but mounting the front sight base does require a drill press. The stubby barrel has a heavy M-4 profile rather than the original skinny version, but it was markedly cheaper. Mounting the front sight base involves seating it in place, locating the holes with a center punch, clamping everything securely, and then drilling the holes for the pins. I broke a couple of drill bits on this chore, as the barrel material was fairly hard. However, mine is essentially a wood shop, not a tool and die facility. The real challenge, however, is that blasted weird buttstock.

You can land a 3D-printed facsimile online for not a lot of money. The decent hard-use replicas of the vintage originals are essentially handmade using original M16 buttstocks cut to length. I found mine on GunBroker, but it was astronomically expensive—like $850. However, originals can easily run into the thousands. Another source is Ordnance Research, which has them for $800.

This replica 607 stock is a great rendition of the original with a few improvements. The locking lever is marked, so you can keep up with whether it is locked or free. The stock internals have also been redesigned to allow mounting without drilling the lower receiver, though the PSA Model 607 lower comes pre-drilled.

As the barrel length is substantially shorter than the legal sixteen-inch minimum, the receiver had to be registered as a short-barreled rifle prior to construction. That process, which involves and ATF Form 1 (and $200), is a royal pain, but the details are available online. While paperwork approval turnaround times in the past have flirted with a year, I have some buddies who recently saw theirs come back after being electronically filed in a week. Weird.

Trigger Time

The end result is surprisingly butt-heavy, particularly with the stubby three-prong flash suppressor in place. The faux moderator is itself fairly substantial and balances the rifle a bit better while directing some of the chaos forward. The gun is extremely compact, and the triangular forearm interfaces nicely with the human form.

Four Targets showing accuracy — The Colt 607 shoots fairly straight given its compact dimensions. These groups were fired from a sandbag rest at 25 yards. (Photo Provided by Author)

The 607 moves like a sports car. The trigger is typical for a rack-grade AR, and the muzzle blast will indeed reliably clear your sinuses. As the 607 was designed as a submachine gun-sized weapon, we did our tactical shooting at 25 yards. We had no stoppages. The gun shot straight for its genre and looks frankly adorable.

Ruminations

Author shooting Colt 607 — The Colt 607 runs like a scalded ape. This compact little gun is incredibly loud, but it is handy and easy to carry, if a bit back heavy. (Photo Provided by Author)

The end result is undeniably expensive if done up properly. However, amidst a sea of modern M4 variants, a period-correct semiauto Model 607 clone SBR will undoubtedly make you some new friends at the local range. The PSA lower has some very cool period markings, and the gun is a bit more palatable with the 3.5-inch muzzle attachment in place. The Luth-AR components form the basis for a quality build. Bizarre, exotic, handy, and just sexy cool, the Colt Model 607 was a steppingstone to much greater things to come.