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.
