Question: BARS replaces a traditional gas piston system,  named after the creator of which weapon?
Context: 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 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 AK-47, or AK as it is officially known (Russian: Автомат Калашникова , "Avtomat Kalashnikova ", 'Kalashnikov's Automatic Rifle' ), also known as the Kalashnikov, is a selective-fire (semi-automatic and fully automatic), gas-operated 7.62×39mm assault rifle, developed in the Soviet Union by Mikhail Kalashnikov.  It is the originating firearm of the Kalashnikov rifle (or "AK") family. The French St. Étienne Mle 1907 (French: "Mitrailleuse Mle 1907 T" ) was a gas operated air-cooled machine gun in 8mm Lebel which was widely used in the early years of the First World War.  The "St.Etienne Mle 1907" was not derived from the Hotchkiss machine gun.  Instead it was an entirely different gas operated blow-forward design borrowed from the semi-automatic Bang rifle of 1903.  This Bang system was first transposed in 1905 to the French Puteaux APX Machine Gun which soon proved to be unsatisfactory.  Then, two years later, the Mle 1907 "St-Etienne" machine gun followed as an improved redesign of the "Puteaux" machine gun.  However the Mle 1907 "Saint Etienne" was only a partial redesign : the original blow-forward gas piston, rack-and-pinion system, and bolt mechanism of the Mle 1905 " Puteaux" machine gun had all been kept only slightly modified inside the newer weapon.  Eventually a total of over 39,700 "St-Etienne" Mle 1907 machine guns were manufactured between 1908 and late 1917.  They were widely used by French infantry during the early part of World War I until their replacement by the distinctly more reliable Hotchkiss M1914 machine-gun. R5 RGP (Remington Gas Piston) is an M4 carbine designed and manufactured by Remington Arms.  It is one of many AR-15 type rifles which uses a gas piston operating system in an attempt to improve the reliability of the weapon. 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. 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. Walt Langendorfer from Rhino International Inc, was an American inventor.  He designed a gas piston system in an effort to convert the AR-15 (M16) rifle to a more reliable operating system than the standard direct impingement system. The Robinson Armament Co. XCR is a multi-caliber, gas piston weapon system developed by Robinson Armament Co. for U.S. Special Operations Command (SOCOM) to satisfy the requirements of the SOF Combat Assault Rifle, or SCAR competition, but was disqualified on a technicality due to late delivery of blank firing adapters.  Robinson Armament continued development and the XCR is now being offered to law enforcement, the military and general public.  Deliveries of the rifle began in mid-2006.  The XCR was displayed to U.S. Army officials during an invitation-only Industry Day on November 13, 2008.  The goal of the Industry Day was to review current carbine technology prior to writing formal requirements for a future replacement for the M4 Carbine.
Answer: AK-47