Fabrique Nationale de Herstal. All rifles were equipped with a fixed A2 stock and a Knight's Armament Company 2-stage match grade trigger The upper receivers were flat top style, but unlike the SAM-R and SPR, they did not have extended feed ramps. Barrel: The 1:7 twist, barrel from the M16A2 and A4 were replaced with a stainless steel Douglas Barrels 1:8 twist, 20-inch barrel, with 12 flutes cut into the barrel to reduce weight. The front sight block was installed with 4 set screws instead of two taper pins. The SDM-R retained the A2-style flash hider. Sights and optics: The issued optic was a 4×32 Advanced Combat Optical Gunsight (ACOG) (models TA31F, TA31RCO, TA01, TA01B, or TA01NSN). A Matech Industries 600-meter backup iron sight was also used. Handguard: Daniel Defense DDM4 Rail 12.0 handguard, with an octagonal aluminum collar locking it to the upper receiver. The handguard provides a free-floating Picatinny rail forend. Bipod: A Harris S-L bipod attached to an ARMS #32 throw-lever rail mount was mounted to the underside of the handguard. Since the handguard was free-floating, it did not come into contact with the barrel, and any pressure from the bipod on the handguard did not deflect the barrel. Variants The 82nd Airborne Division examined an alternate version, based on the M4 carbine. The barrel was to have been an long fluted Douglas barrel with 1:8 twist. A mid-length gas system was to be used, along with the Daniel Defense M4Rail 9.0 handguard. This effort never went beyond the staffing process. See also United States Marine Corps Squad Advanced Marksman Rifle United States Marine Corps Designated Marksman Rifle SEAL Recon Rifle Mk 12 Special Purpose Rifle References External links Field Manual 3-22.9: Squad Designated Marksman training 5.56×45mm NATO semi-automatic rifles Designated marksman rifles Rifles of the United States ArmaLite AR-10 derivatives Military equipment introduced in the 2000s