6.5mm Grendel

The name "Grendel" is inspired by the mythical monster antagonist from the Old English epic poem Beowulf. It was a trademark owned by Alexander Arms (Bill Alexander's company in Radford, Virginia) and manufactured at Radford Arsenal, until legally released in 2010 for SAAMI standardization with collaboration from Hornady.

• Name — 6.5mm Grendel • Origin — United States • Type — Rifle • Designer — Bill Alexander and Janne Pohjoispää • Design Date — 2003 • Parent —.220 Russian (5.6×39mm) • Case Type — Rimless, bottleneck • Is Si Specs — yes • Bullet — 6.71 • Land — 6.50 • Neck — 7.44 • Shoulder — 10.87 • Base — 11.15 • Rim Dia — 11.2 • Rim Thick — 1.5 • Case Length — 38.7 • Length — 57.5 • Case Capacity — 2.3 • Max Pressure — 360 • Rifling — 1 in or 1 in • Primer — Small rifle • Bw1 — 90 • Btype1 — Speer TNT • Vel1 — 2880 • En1 — 1658 • Bw2 — 108 • Btype2 — Scenar (moly) • Vel2 — 2790 • En2 — 1866 • Bw3 — 120 • Btype3 — Norma FMJBT • Vel3 — 2700 • En3 — 1942 • Bw4 — 123 • Btype4 — Sierra Matchking • Vel4 — 2650 • En4 — 1917 • Bw5 — 130 • Btype5 — Norma • Vel5 — 2510 • En5 — 1818 • Test Barrel Length — 24 in • Balsrc — Alexander Arms Pressure-safe Load Data

The goal of the 6.5mm Grendel design was to create an effective STANAG magazine-length cartridge for the AR-15 platform that could reach and surpass the performance of the native 5.56mm NATO/.223 Remington cartridge. Constrained by the dimension of the STANAG magazines, the Grendel's designers decided to use a shorter, larger-diameter case for higher powder volume while allowing space for the long, streamlined, high ballistic coefficient 6.5mm (.264 cal.) bullets. Firing factory-loaded ammunition with bullets ranging from 90 to, its muzzle velocity varies from with 129- and 130 gr bullets to with bullets (similar in velocity to a 77 gr 5.56 mm round). Depending on their case material and bullet weight, 6.5mm Grendel cartridges weigh 14.7 to. That is significantly more when compared to 8 to 12 grams (120 to 190 gr) 5.56mm NATO rounds. The case head diameter of the Grendel is the same as that of the 5.6×39mm (.220 Russian), 7.62×39mm and 6.5mm PPC cases. This diameter is larger than the 5.56×45mm NATO, thereby necessitating the use of a non-standard AR-15 bolt. The increased case diameter results in a small reduction in the magazine ammo capacities. A 6.5mm Grendel magazine with the same dimensions as a 30-round STANAG magazine will hold 26 rounds of Grendel ammunition.

• Illustration — C-Products 26-round Grendel Magazine Proponents assert that the Grendel is a good "middle ground" between the 5.56×45mm NATO and the 7.62×51mm NATO. It retains greater terminal energy at extended ranges than either of these cartridges due to its higher ballistic coefficient (BC). For example, the 6.5 mm Grendel bullet has more kinetic energy and better body armor penetration at than the larger and heavier bullet of the M80 7.62mm NATO round. In order to obtain ballistics that are superior to the 7.62×51mm cartridge, a weapon with a longer barrel and firing a heavier bullet is necessary. To achieve the same results from shorter-length barrels, even heavier bullets are needed.

• Illustration — Muzzle Velocity Change with Bullet Weight • Bullet velocity: 24 inch (609.6 mm) barrel • Bullet mass — Muzzle velocity — 1,000 meter velocity • gr — g — ft/s — m/s — ft/s — m/s • Lapua Scenar — 108 gr — 2700 ft/s — 1166 ft/s • Lapua Scenar — 123 gr — 2620 ft/s — 1222 ft/s • Lapua FMJBT — 144 gr — 2450 ft/s — 1213 ft/s As noted above, the Grendel case is very closely related to the.220 Russian case. In general, each additional grain of bullet weight will reduce muzzle velocity by 10.8 ft/s (50.8 m/s for each gram) and each additional inch of barrel length will increase muzzle velocity by 20 ft/s (2.4 m/s for each centimeter). Specific details are available as graphs derived from Alexander Arms' public domain load table linked below.

Serbia has adopted two rifles made by Zastava Arms in 6.5 mm Grendel caliber as main armament for its armed forces. The two rifles, designated Zastava M19 (officially adopted by the Serbian Armed Forces in 2022) and Zastava M20 (a compact carbine variant of the M19), are derivatives of the previous-issue M70 rifle. Serbia is also currently testing a light machine gun chambered in 6.5 Grendel based on their current Zastava M84 general purpose machine gun which is chambered in 7.62×54mmR. An American-manufactured rifle in 6.5mm Grendel caliber may also be adopted in armament for special forces units after it passes testing in Technical Testing Center. Three types of 6.5mm Grendel ammunition produced by Prvi Partizan of Užice, Serbia, will be tested for use with these rifles. The French police tactical unit GIGN announced in 2025 that they are partnering with Swiss ammunition maker SwissP Defenc to produce 6.5 Grendel cartridges for the GIGN.