.277 SIg Fury or 6.8×51mm Common Cartridge, (designated as the .277 SIG Fury by the SAAMI) is a centerfire rimless bottlenecked rifle cartridge announced by SIG Sauer in late 2019.Its hybrid three-piece cartridge case has a steel case head and brass body connected by an aluminum locking washer to support the high chamber pressure of 80,000 psi (551.6 MPa)
The cartridge was designed by SIG Sauer for the United States Army’s Next Generation Squad Weapon Program (NGSW). It is dimensionally similar to the 7.62×51mm NATO service cartridge.
The cartridge was announced for non-military usage along with the SIG Sauer CROSS—a bolt-action, magazine-fed rifle—in December 2019. As a short-action rifle cartridge (cartridges having an overall length of 2.750 inches (69.85 mm) or less), increased internal ballistic performance for its cartridge case volume is accomplished by applying high gas pressure.In 2020, it was accepted by the Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) as a new cartridge and chambering.In 2022 SIG Sauer announced that it intends to commercially chamber the SIG MCX Spear semi-automatic rifle in .277 Sig Fury.
The cartridge uses a case that is the same length and diameter as the .308 Winchester. Each “hybrid” cartridge case consists of a stainless steel base coupled to a brass body via an aluminum locking washer.Stainless steel has a significantly higher yield strength than brass, allowing the engineers to use higher maximum average pressure (MAP) chamber pressure levels. Bullets are either 135 grains (8.75 g) “match grade” with a G1 ballistic coefficient of ≈ .488 or 140 grains (9.07 g) “hunter tipped” with a G1 ballistic coefficient of ≈ .508 (ballistic coefficients are somewhat debatable).These projectile ballistic coefficients are equal to the publicized specifications of the Sierra .277 135 grains (8.75 g) HPBT MatchKing and .277 140 grains (9.07 g) TGK GameKing projectiles.The SAAMI warns that MAP levels greater than 65,000 psi (448.2 MPa) may present an increased risk of unsafe cartridge case or firearm rupture and thus require cartridge case and firearm designs that depart from traditional practices (materials, construction, and other design criteria).The .277 Fury SAAMI (voluntary) MAP chamber pressure of 80,000 psi (551.6 MPa) enables a 135 grains (8.7 g) projectile muzzle velocity of 3,000 feet per second (914 m/s) from a 16-inch (406 mm) barrel.It also means that the .277 Fury is normally chambered in small arms that are capable of handling the accompanying bolt thrust safely.