The M6 is a series of carbines designed and manufactured by LWRC International.
 It is based on the M4 carbine, with which it shares 80% of its parts.
 The 'M' model name is not a US military designation.
 Like the HK416, it features a proprietary short-stroke self-regulating gas piston system and bolt carrier/carrier key design, which prevents trapped gases from contacting the bolt carrier or receiver of the weapon, which reduces the heating and carbon fouling of the internals, simplifies field maintenance, and improves reliability.
The SIG MCX is a weapon series designed and manufactured by SIG Sauer, featuring a short-stroke gas piston system, carried over from the SIG MPX submachine gun.
 It is made in both semi-automatic and selective fire configurations.
 It is available in rifle, short-barreled rifle, and pistol configurations.
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 Heckler & Koch HK416 is an assault rifle/carbine designed and manufactured by Heckler & Koch.
 Although its design is in large part based on the AR-15 class of weapons, specifically the Colt M4 carbine family issued to the U.S. military, it uses an HK-proprietary short-stroke gas piston system originally derived from the ArmaLite AR-18 (the same system was also used in Heckler & Koch's earlier G36 family of rifles).
 The HK416 gained fame as the weapon that United States Navy SEALs from DEVGRU Red Squadron used to kill Osama Bin Laden in 2011.
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 AL-7 is a Russian assault rifle designed by Izhmash engineer Yury Aleksandrov in the early 1970s.
 The AL-7 uses a type of operation developed by Pyotr Tkachyov of TsNIITochMash (Central Institute for Precision Machine Building) in the mid-1960s known as "Balanced Automatics" first used on the AO-38 assault rifle.
 The Balanced Automatics Recoil System (BARS) replaces the traditional Kalashnikov gas piston operating system, reducing the negative effects of recoil and allowing more efficient use of automatic fire.
 BARS works by shifting mass toward the muzzle of the rifle as the bolt and bolt carrier recoil rearward.
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 SIG MPX is a gas-operated submachine gun designed and manufactured by SIG Sauer, and is primarily chambered in 9×19mm Parabellum cartridge.
 It is the first submachine gun to feature a fully closed and rotating bolt and piston system.
 This was done to enhance the safety of the user and to have a more reliable firearm.
 It was designed in 2013 and was released to the general public in 2015.
 It features the SIG Sauer short stroke push-rod gas system to reduce the recoil and improve the reliability of the weapon.
The STG-556 is a Steyr AUG A1 a clone manufactured by Microtech Small Arms Research (MSAR).
 It was introduced at the 2007 SHOT Show.
 The STG-556's working system features a bolt release as seen on the M16 rifle; otherwise, the STG-556 retains the features similar to the Steyr AUG, such as having the quick-change barrel option and the ability to change which side the weapon ejects from by changing the bolt out for a right or left side bolt respectively.
 There is some parts interchangeability but the rifle is not a true clone.
 The STG-556 ships standard with right side ejector.
 The STG-556 is unable to use M16-style magazines or STANAG NATO standard magazines; they use versions of a proprietary MSAR magazine resembling the Steyr mags.
 The current production XM-17 E4 use STANAG magazines.
 The STG-556 is a short-stoke gas piston.
 The STG-556 rifle can be converted from either having a telescopic sight or a MIL-STD-1913 rail.
 It is available in civilian (semi-automatic only) and military/law enforcement (select-fire) variants.
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.

Question: Which gas-operated (short-stroke gas piston) self-loading rifle with a rotating bolt is developed by the Tangodown company?
Answer: FN SCAR