About a dozen years ago a friend told me that Crane Naval Station (USSOCOM) was looking into a quick-change barrel system for the M4, and he asked for my opinion. When I told him they ought to look at the system Eugene Stoner came up with, he asked what that was. Stoner used two push-pins to hold the receivers together and, instead of changing the barrel, one could instantly change out the entire upper component for another with a longer or shorter barrel, and it could even be pre-sighted in. Of course, I was having some fun with this, as my friend is extremely knowledgeable about firearms, but he replied, “You know, you’re right!”
Not long after that conversation, quick-change barrels for the AR platform did appear, the most notable being those from Lewis Machine & Tool and MGI. Others have followed, but replacing the upper receiver group of the AR arguably remains one of the best solutions, and the concept originated with Eugene Stoner himself.
In 1957, Colt bought the entire AR package from Eugene Stoner, who owned the patent on the direct gas “transfer” system of operation when he went to work for Armalite. This package included both the final version of the 7.62x51 mm NATO (.308 Win.) AR-10 and the AR-15 that fired the .223 Rem. cartridge designed by Armalite’s L. James Sullivan and Robert Fremont. Sullivan and Fremont also designed the AR-15 rifle for Stoner, and although Colt was preparing to manufacture the AR-10 for the 7.62x51 mm cartridge, the demand quickly changed for the smaller AR-15 in .223 (later the 5.56x45 mm NATO), and Colt shelved the idea of making any version of the AR-10. Now, more than a half-century later, that has changed in a very big way.
Called the LE901-16S Modular Carbine, Colt’s new 7.62x51 mm NATO is the result of an out-of-the-box concept that is a marriage of the AR-10 and AR-15 platforms with far-reaching implications. In a nutshell, the LE901-16S is an adaptive rifle using a highly modified and upgraded AR-15 lower receiver with an equally unique AR-10 magazine well. As such the rifle quickly converts to an M4-type carbine in 5.56x45 mm NATO.
The LE901-16S comes with a four-prong flash hider on a 16.1-inch, four-groove, 1:12-inch right-hand twist barrel. A barrel-mounted front sight is generally preferred and, along with a bayonet lug, the LE901’s elevation-adjustable folding front sight is part of the gas block, and is elevated under detent pressure, but requires pushing up the front lock to fold down. The rear sight is a flip-up dual aperture that is adjustable for windage using the tip of a bullet. A left side button is depressed to unlock it.
A Monolithic Upper Receiver
In all AR-type rifles, the alloy upper receiver suffers to some degree as the bolt carrier rotates the steel bolt cam into alignment with its guide channel. This occurs as the head of the cam pin bumps the corner of the channel when the bolt unlocks. To prevent this in the LE901-16S, Colt has semi-permanently imbedded a hardened steel implant into the upper receiver, and has rounded the corners of the head of the cam pin.
Compared with the M4, the LE901’s bolt and components are massive, and its firing pin has a return spring to prevent inadvertent firing from kinetic energy. Most other aspects of the rifle are standard, except for its Vltor EMOD adjustable buttstock with dual battery compartments, three sling points and a rubber buttpad. The rifle also comes with two 20-round 7.62x51 mm NATO PMAGs from Magpul Industries.
While the front 2.68 inches of the LE901 carrier is 1.185 inches in diameter, the last 4.5 inches of the carrier measures 0.93 inches in diameter, the same as the end of the carrier of an AR-15. This section is also off center (lower) from the larger front portion. This means that the standard AR-15 hammer can be used and that the carrier will work in a standard AR-15 size recoil spring tube and will accommodate any mil-spec M4 type buttstock. On the bottom of the rear of the carrier are raised pads to prevent bolt carrier tilt.
Although the LE901-16S’ upper receiver group looks pretty much like other AR-10 variants, its longer than normal pivot pin hinge is quite noticeable, and the rifle’s lower receiver group is also full of surprises. This is not only because of its ambidextrous controls, but more so because of what is in front of these parts, and that is what sets this rifle apart from all others.
Rapid Caliber Conversion
After putting the safety “on,” removing the magazine and making sure the chamber is empty, removing the LE901’s 7.62x51 mm upper receiver in the normal manner allows one to replace it with a Colt LE6920CK 5.56x45 mm NATO upper receiver component by first adding the three necessary conversion parts. These include the AR-15 magazine well adapter and its captive cross pin, the M4 recoil spring and the standard AR-15 “H” buffer/guide.
To accomplish the conversion, simply attach the magazine well adapter to the 5.56 mm upper receiver’s front pivot hole by pushing adapter’s captive pin through the hole. Then insert the magazine well adapter into the top of the LE901’s magazine well and attach the upper receiver in the normal manner, pushing the lower hinge pin through the magazine well’s attachment hole. Then, with the hammer still cocked, close the upper and lower receiver and depress the rear locking pin. The converted rifle is ready to use as a 5.56x45 mm NATO rifle. The upper will now open just enough to remove the bolt group.
The average time needed to complete the conversion to or from either caliber should be less than two minutes. If you’re wondering if the conversion can be made using other makes and models of AR-15 type upper components, the answer is typically yes. With roughly 50 companies now offering “clones” of AR-15 rifles and carbines, not all are created equally. I was able to convert my sample Colt 7.62x51 mm LE901-16S to 5.56x45 mm using six different AR-15-type upper receiver groups with one being only slightly difficult. This can happen because not all AR-15 clones are made to mil-spec. Only the test results using the two Colt upper receiver components are shown here.