Originally posted by Yale
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Upper and lower receivers: The rifles were produced by the Army Marksmanship Unit (AMU) at Fort Benning, Georgia using either new semi-automatic ArmaLite lower receivers or existing Army M16A2 or A4 lower receivers previously supplied by either Colt or Fabrique Nationale de Herstal. All rifles were equipped with a fixed A2 stock and a Knight's Armament Company 2-stage match grade trigger capable of only semi-automatic fire. 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, 20-inch (510 mm) 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 4x32 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 M4 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 #17 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.
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