To withstand the added power of the .450 Bushmaster, the AR-556 MPR is built with a strong, cold hammer-forged barrel measuring 18.63 inches long and made from 4140 chrome-moly steel. Complete with five-groove rifling, the barrel features a twist rate of 1:16 inches designed to stabilize .45-caliber rounds weighing between 170-260 grains. For enhanced accuracy, the barrel features 5R rifling that cuts down on both bullet deformation and fouling in the sharp corners between lands and grooves, allowing the rifle to maintain its accuracy, even with carbon buildup.
The barrel extension is paired with a bolt machined from high-strength steel alloy with 50 percent more strength than comparable AR bolts. Each bolt is machined with tapered lugs that provide added support to the larger cartridge case, and the bolts are pressure-tested to ensure their ability to withstand the pressures of the .45-caliber cartridge. At the muzzle-end of the barrel, Ruger added a radial-style muzzle brake that takes the edge off the recoil of the .450 Bushmaster round. The 11/16-24 TPI threads at the muzzle are also compatible with a range of other .45-caliber muzzle devices, such as brakes and sound suppressors.
A carbine-length gas system provides the best-possible reliability with most types of .450 ammunition on the market, and the entire system is surrounded by a free-float handguard measuring 15 inches in length. The handguard provides users with M-Lok attachment points along its entire length at the 3-, 6-, and 9-o'clock positions.
Inside the forged-aluminum receiver, consumers will find the Ruger AR-556 equipped with the company's Elite 452 trigger, a two-stage design providing a smooth pull weighing just 4.5 pounds. The trigger assembly is constructed with a lightened hammer that offers a lock time measuring 30 percent faster than mil-spec designs.