The Bushmaster ACR is a firearm both propelled and burdened by its legacy as the Magpul Masada. First unveiled at the 2007 SHOT Show in Orlando, Fla., the gun created quite a stir among the semi-automatic rifle aficionados in attendance. The Masada promised to deliver first-rate ergonomics and versatility in the form of its quick-change barrel capability and adaptability to alternate calibers.

Magpul had something good but it didn’t have the resources to produce the gun, so in 2008 it sold the design to Bushmaster, which re-named the firearm Adaptive Combat Rifle (ACR). Bringing a gun like the Masada to market would be a tough job, but Bushmaster and Remington, both members of the Freedom Group family of firearm companies, shared their resources and unique expertise to make the promise of the Masada a reality. Magpul may have sold its rights to the gun, but it remained heavily involved in the development of the ACR.

Bushmaster and Remington will both sell ACRs. Bushmaster will handle civilian and state and local law enforcement sales, while Remington will handle potential ACR sales to federal law enforcement and the military. Bushmaster will offer two models, the Basic and the Enhanced, with the only significant differences being the stock and the fore-end. The Basic will have a fixed stock and polymer fore-end and the Enhanced will have a folding stock and a quad-rail aluminum fore-end. I received an Enhanced ACR for examination and testing.

A Better Mousetrap
The ACR dispenses with the direct gas-impingement operating system of the M16 in favor of an AR-180-style indirect gas-impingement system. Gas tapped from a port in the barrel travels through the gas block and impinges on a tappet rod above the barrel. Energy from the expanding propellant gases drives the tappet rod back. The tappet rod in turn strikes the face of the bolt carrier to unlock the bolt.

The ACR has a short-stroke rotary bolt with eight equally spaced locking lugs that engage an extension on the breech end of the barrel. Seven of the lugs have the same height, width and depth, while a smaller eighth lug is part of the extractor. A plunger ejector can be found on the recessed bolt face. There are no gas rings on the bolt body, as it’s not a direct-gas gun, so it doesn’t need them. A coil spring wrapped around the steel firing pin reduces the chances of a slam fire. Rotation of the bolt is controlled by a cam pin that travels in a kidney shaped cut-out in the left side of the carrier.

The bolt carrier slides on a single action rod that controls the compression of the recoil spring. A white polymer buffer is fixed to the tail of the action rod. U-shaped steel guide rails anchored to the aluminum upper receiver by machine screws control the vertical and horizontal movement of the bolt carrier as it travels back and forth during the firing cycle.

The advantages of the ACR’s operating system are: It has no need for a separate buffer extension, so it is compatible with a folding stock; the travel of the blocky bolt carrier is less likely to be impeded by dirt or accumulated fouling in the upper receiver; and much less fouling and heat is vented into the bolt and action mechanism during firing. Most of the fouling is trapped in the gas block. Thankfully, the two-position gas regulator, along with the tappet rod and action spring, can be removed for cleaning. The two-position regulator can be rotated in either a clockwise or counter-clockwise direction. The “S” setting is for firing with an attached suppressor and “U” setting is for normal unsuppressed operation.

Quick-Change Barrel
The standard barrel length for both the Basic and Enhanced models is 16 1/2 inches. Remington representatives told me that their internal testing determined the barrel’s ideal thickness and stepped profile for maximum longevity. The 10 1/2- and 14 1/2-inch-barreled guns will be available to law enforcement customers and Bushmaster will offer 18-inch-heavy barreled guns as Designated Marksman-style rifles. Most of the shooters I know who are eager to buy an ACR will likely want to shoot it with 75- and 77-grain loads, so the 1:9-inch rifling twist of our test gun seems to be an odd choice. Bushmaster told me that 1:9 inches was the most popular choice among its distributors, but I was assured that guns with 1:7-inch barrels will be available, and separate 1:7-inch barrels will be available through a spare barrel program. Also 6.8x43 mm barrels with Spec. II chambers will be introduced in the same lengths. All ACR barrels in both chamberings are cold-hammer forged with six grooves in a right-hand twist.

A barrel nut with interrupted threads locks the barrel assembly into a steel trunnion fixed to the upper receiver by way of two roll pins and two machine screws. A hinged barrel wrench attached to the barrel nut makes it easy to dismount the barrel for cleaning or swapping calibers. It should be noted that a caliber change will also require replacing the bolt and magazine.

To change the barrel, simply lock the bolt to the rear. Knock out the captured ferule pin at the bottom rear of the aluminum fore-end. Pull the U-shaped fore-end forward and off of the gun. Pull down on the hinged barrel wrench and rotate the barrel wrench/barrel nut lever one-quarter turn to the left. Then unseat the barrel nut from the trunnion by pulling forward on the barrel wrench. The barrel assembly and the attached gas system assembly can then be removed.

Construction
The upper receiver is a U-shaped, one-piece assembly of extruded aluminum that extends from the wrist of the stock to the gas block. An integral M1913 Picatinny rail runs the full length of its roof. A shell deflector is mounted behind its large ejection port. The upper receiver also serves as a housing for the reversible charging handle, the bolt, and the steel trunnion that provides a common joint for the ACR’s upper and lower receivers and its detachable barrel.

Magpul specializes in polymer accessories for the AR-15/M16 platform, and the ACR’s polymer lower receiver is a legacy of its Magpul origins. The lower receiver consists of an integral pistol grip, trigger well and magazine well. All of the fire and function controls including the trigger, ambidextrous safety lever, ambidextrous magazine release and bolt stop buttons are contained in the ACR’s lower receiver. It also serves as a mounting point for the buttstock, which is held in place by both a tongue-and-groove joint and a captured steel ferule pin at the rear of the assembly. Like the M16, the upper and lower receivers are hinged on a captured pivot pin in front of the magazine well. Unlike the M16, the rear take-down pin is housed in the upper rather than the lower receiver.

The skeletonized polymer stock of the ACR Basic is fixed with a 12-inch length of pull that can be extended with compatible inserts from Magpul. In contrast, the stock of the Enhanced is adjustable in every way imaginable. The comb can be raised a 1/2 inch and the user can choose one of seven settings for length of pull, ranging from 11 to 14 inches in 1/2-inch increments. A push-button release on the left side of the stock near the wrist allows the stock to fold over the right side of the receiver. When folded, it is held in position by tension but doesn’t really lock. This is something of a compromise that allows it to lock back in its extended firing position without pushing any buttons. When folded, the stock does not block the ejection port, but shooters may find that it impairs access to some of the left-handed controls on the right side of the receiver including the safety lever and magazine release button. Battery storage compartments are included in the stock and pistol grip. They are compatible with most of the Magpul MIAD Grip Core inserts.