The L1A1 Self-Loading Rifle, also known as the SLR (Self Loading Rifle), by the Canadian Army designation C1A1 (C1) or in the US as the "inch pattern" FAL.
 It is a British Commonwealth derivative of the Belgian FN FAL battle rifle ("Fusil Automatique Léger" ["Light Automatic Rifle"] produced by the Belgian armaments manufacturer Fabrique Nationale de Herstal [FN]).
 The L1A1 is produced under licence and has seen use in the Australian Army, Canadian Army, Indian Army, Jamaica Defence Force, Malaysian Army, New Zealand Army, Rhodesian Army, South African Defence Force and the British Armed Forces.
TangoDown is an American manufacturer and marketer of firearms accessories.
 It developed the ARC magazine for the SCAR program with assistance from United States Navy SEALs.
 It also produces the "BattleGrip" foregrip and other accessories.
The A-91 bullpup assault rifle "(also known as A-91M)" was developed during the 1990s by KBP Instrument Design Bureau in Tula, as an offspring of the A-91 family of compact assault rifles described in the 9A-91 article.
 While the A-91 retains the basic gas-operated, rotating bolt action and a trigger unit design from 9A-91, it features a bullpup polymer housing, with an integral 40 mm single-shot grenade launcher mounted under the barrel.
 The earliest prototypes of the A-91 bullpup were fitted with the grenade launcher above the barrel, and with a front vertical foregrip; current models are fitted with the underbarrel launcher, which also serves as a forearm.
 The A-91 features a forward ejection system, initially developed in Tula by designers like Afanasiev during the early 1960s.
 In this system, the ejection port is located above the pistol grip, and points forward.
 Extracted cases are fed from bolt head through the short ejection tube to the ejection port, and fall out of the gun well clear of the shooter's face, even when firing from the left shoulder.
 As for now, the A-91 is made in small number and, probably, is used by some elite law enforcement units in Russia; it is also offered for export and domestic military and police sales.
The Jarmann M1884 fired a 10.15 mm black powder cartridge in an 8-round, tubular magazine in which the rounds were lined up in a tube below the barrel.
 It has a non-rotating bolt (the part of the action that seals the rear end of the barrel) locked by a rotating bolt handle, and reputedly a smooth action.
 However, this action is not considered strong enough to fire modern ammunition, since the only locking is provided by the rotating bolt handle.
A semi-automatic rifle, also known as a self-loading rifle ('SLR') or auto-loading rifle, is a self-loading rifle that fires a single round each time the trigger is pulled.
 In contrast, a fully automatic rifle continuously fires rounds as long as the trigger is pressed and there is ammunition left.
The Saiga-12 ( ) is a 12-gauge shotgun available in a wide range of configurations, patterned after the Kalashnikov series of rifles and named for the Saiga antelope.
 Like the Kalashnikov rifle variants, it is a rotating bolt, gas-operated gun that feeds from a box magazine.
 All Saiga-12 configurations are recognizable as Kalashnikov-pattern guns by the large lever-safety on the right side of the receiver, the optic mounting rail on the left side of the receiver and the large top-mounted dust cover held in place by the rear of the recoil spring assembly.
The ArmaLite AR-15 is a selective-fire, 5.56×45mm, air-cooled, gas-operated, magazine-fed rifle, with a rotating bolt and straight-line recoil design.
 It was designed by Eugene Stoner and it is based on the Armalite AR-10 rifle.
 The AR-15 was designed above all else to be a lightweight assault rifle, and to fire a new lightweight, high-velocity small caliber cartridge to allow the infantrymen to carry more ammunition.
Rotating bolt is a method of locking used in firearms.
 Johann Nicolaus von Dreyse developed the first rotating bolt firearm, the "Dreyse needle gun" in 1836.
 The Dreyse locked using the bolt handle rather than lugs on the bolt head like the Mauser M 98 or M16.
 The first rotating bolt rifle with two lugs on the bolt head was the Lebel Model 1886 rifle.
 The concept has been implemented on most firearms chambered for high powered cartridges since the 20th century.
The FN SCAR (Special Operations Forces Combat Assault Rifle) is a gas-operated (short-stroke gas piston) self-loading rifle with a rotating bolt.
 It is constructed to be extremely modular, including barrel change to switch between calibers.
 The rifle was developed by FN Herstal (FNH) for the United States Special Operations Command (SOCOM) to satisfy the requirements of the SCAR competition.
 This family of rifles consist of two main types.
 The SCAR-L, for "light", is chambered in 5.56×45mm NATO cartridge and the SCAR-H, for "heavy", is chambered in 7.62×51mm NATO.
 Both are available in "Close Quarters Combat" ("CQC"), "Standard" ("STD") and "Long Barrel" ("LB") variants.
The Heckler & Koch HK417 is a battle rifle designed and manufactured by Heckler & Koch in Germany.
 It is a gas-operated, selective fire rifle with a rotating bolt.
 It is chambered for the full-power 7.62×51mm NATO round, instead of the less powerful 5.56×45mm NATO intermediate cartridge.
 The HK417 is intended for use in roles where the greater penetrative power and range of the 7.62×51mm NATO round are required.
 It has been adopted for service by a number of armed forces, special forces, and police organizations.

Question: Who assisted the manufacturer that developed the ARC magazine for the gas-operated self-loading rifle with a rotating bolt?
Answer: United States Navy SEALs