The addition of an embedded marksman at the platoon or squad level has been a continuing process in the U.S. military. The U.S. Marine Corps experimented with this during Project Metropolis, before creating the "Squad Advanced Marksman" position along with the Squad Advanced Marksman Rifle specifically for this purpose. The 3rd Infantry Division followed suit in implementing a training program to have one marksman per squad, the Squad Designated Marksman, and developing a rifle, the Squad Designated Marksman Rifle. The SDM is an integral part of the squad and serves as a rifleman first and designated marksman second. The SDM is not meant to be a squad sniper who engages the enemy with precision fire at long ranges, but instead is trained to directly support the squad with well-aimed shots at ranges slightly beyond the normal engagement distances for riflemen. Much like the U.S. Marine Corps SAM-R, the 3rd Infantry division SDM-R was an accurizedM16 rifle built in-house by the U.S. Army Marksmanship Unit, and was informally known as "the AMU rifle". SDM-Rs have only been built for one major Army division and is not an Army program of record. The SDM-R is designed to provide engagement capability at the squad level to 600 meters, similar to the needs expressed by members of USSOCOM that led to the development of weapons such as the United States Navy Mark 12 Mod X Special Purpose Rifle and the SEAL Recon Rifle. The National Guard Marksmanship Training Center of the Arkansas Army National Guard conducts the fourteen-day-long Squad Designated Marksman Course at Camp Joseph T. Robinson. Soldiers fire over 1500-rounds from the M16A4 rifle under the close supervision of course cadre. Task Force Small Arms Readiness Group conducts the week-long SDM course at the SARG academy at Camp Bullis, Texas. Many of the instructors are President's 100 recipients.
Specifications
The US Army used either 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 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.
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 #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.
