6.5 mm Grendel

The 6.5 mm Grendel (or 6.5x39mm Grendel) is a 6.5 mm caliber intermediate cartridge developed in 2002 by Bill Alexander of Alexander Arms and Arne Brennan of Competition Shooting Sports. This cartridge is an evolution of Dr. Lou Palmisano's PPC case which has dominated competition shooting for over 20 years. The 6.5 Grendel was designed as a low recoil, high accuracy, long-range cartridge for the AR-15 platform.

Development and history


The Grendel design philosophy was "start slow and end fast". High muzzle velocities accelerate barrel wear and increase the percentage of recoil due to escaping gases but only yield good impact energy if the bullet is efficient at carrying its velocity downrange. Constrained by the length of the 5.56 mm NATO round, but wanting to launch a much heavier bullet, the Grendel designers hit on a short, fat case for more powder volume while saving space for long, streamlined, high ballistic coefficient (BC) bullets that give up little of their energy in-flight. Firing factory bullets from 90 to 129 grains (5.8–8.4 g), its muzzle velocity ranges from 2500 ft/s with 129- and 130 gr bullets to 2900 ft/s with 90 gr bullets (similar in velocity to a 5.56 mm 77 gr round).

Timeline

 * 1998: Arne Brennan investigates .22 PPC, 6 PPC in search of a more capable competition and hunting round.
 * 2000: Arne Brennan begins shooting a 6.5 PPC wildcat cartridge from his custom AR-15 platform.
 * 2001: Arne Brennan and Bill Alexander introduced to each other by Lothar Walther.
 * Mar 2002: First production 6.5 Grendel brass cartridges manufactured to specification by Lapua.
 * May 2003: First prototype 6.5 Grendel weapon demonstrated at Blackwater Shoot-out.
 * May 2006: Independent ballistic gelatin testing completed for 90 gr TNT, 120 gr Norma, 120 gr SMK, and 123 gr SMK prototype.
 * Aug 2006: Bill Alexander publishes pressure safe load table for AR platforms with 14.5- to 28.0-inch (370–710 mm) barrels.
 * Feb 2007: Production Wolf brand ammunition becomes available.

Performance
Proponents assert that the Grendel is an ideal middle ground between the 5.56 mm NATO and the 7.62 mm NATO, taking the best attributes of each. It has a flatter trajectory and retains greater terminal energy at extended ranges than either of these cartridges due to its higher ballistic coefficient. Production rifles have consistently achieved sub–minute of arc (MOA) groups at test ranges out past 600 meters. On March 6, 2006, Arne Brennan achieved a witnessed 1.198 in group at 660 yd. Similar accuracy was demonstrated at recent Blackwater Shootouts. Competitions have begun to be won with the 6.5 Grendel although use of the round is still not common in competition.

External ballistics


As noted above, the Grendel case is very closely related to the PPC case. When sufficient load data was made available, a thorough study of the Grendel case, which constitutes a precisely dimensioned combustion chamber when the round is chambered, was done with the following results. In general, each additional grain of bullet weight will reduce muzzle velocity by 10 ft/s (3 m/s) and each additional inch of barrel length will increase muzzle velocity by 20 ft/s (6 m/s). Therefore, a handy rule of thumb is "one inch of barrel length equals two grains of bullet weight (1 mm → 5 mg)". Specific details are available as graphs derived from Alexander Arms' public domain load table linked below.

Terminal ballistics
Alexander Arms contracted Speer Law Enforcement to perform terminal ballistics tests in accordance with standard F.B.I. Protocols and their standard in-house procedures. They use 6 in × 6 in × 16 in (150 mm × 150 mm × 400 mm) blocks of ten percent ballistic gelatin, calibrated with a BB. These results were made public in May of 2006. All tests were completed using either 14.5- or 16.0-inch (370 or 400 mm) chrome-lined Alexander Arms Tactical rifles and shot from either 50 or 100 yards (45 or 90 m) to simulate combat conditions using short-barreled M4 format weapons. Barrel pressures were less than 345 MPa (50,000 psi). These are all production rifles and ammunition except for the prototype 123 gr Sierra MatchKing (now a production bullet). In addition to the photos shown below, a 90 gr TNT was tested resulting in explosive fragmentation after penetrating only 0.5 in (13 mm).

The 120 gr Sierra MatchKing penetrated 3.25 in before yawing and fragmenting. The Alexander Arms Tactical 16 carbine (16 in barrel) was used at a range of 100 yd. Impact velocity: 2383 ft/s. Maximum penetration of the 120 gr SMK was 19.5 in, maximum permanent cavity diameter was more than 6 in with lesions running to gel block exterior surfaces. Depth to the maximum permanent cavity was 7.5 in. The bullet fragmented, with seven large pieces visible within the block. Jacket sections came to rest at 11.75 and 16.25 inches (298 mm and 413 mm), and the bullet core at 19.5 in.

The prototype 123 gr SMK penetrated 2 in before yawing and fragmenting. The Alexander Arms Tactical 14.5 carbine (14.5 in barrel) was used at a range of 50 yd. Impact velocity was 2385 ft/s. The 123 SMK penetrated to a depth of 16.2 in, maximum permanent cavity diameter was more than 6 in with lesions running to block exterior surfaces. Depth to the maximum permanent cavity was 7 in. The bullet fragmented into multiple small fragments with jacket pieces visible at 11 and 13 inches (280 and 330 mm). A small core fragment was visible at a maximum depth of 16.2 in.

The 120 gr Norma FMJ penetrated 16.5 in before veering out the side of the block and impacting the support frame. No fragmentation was evident, but the slug is believed to have tumbled at about 7 in with its maximum permanent cavity at 11 in. Lesions of more than 6 in were torn through the top and bottom block surfaces. The bullet was fired from an Alexander Arms Tactical 14.5 in AR-15 rifle at a range of 50 yd with a chronographed impact velocity of 2405 ft/s.

Periodical References (with no online copy)

 * A. Guns 'n' Ammo: Book of the AR-15, 2004, "The 6.5mm Grendel," David Fortier, p. 66.
 * B. Special Weapons for Military & Police, Annual #27 2004, "Beyond the 5.56mm NATO," Stan Crist, p. 62-67.
 * C. Guns 'n' Ammo: Book of the AR-15, 2005, "6.5 Grendel and 6.8 SPC," David Fortier, p. 32-44.
 * D. Shooting Times, February 2005, "Cooking up Loads for the 6.5 mm Grendel," David Fortier, p. 52-56.
 * E. Shooting Illustrated, September 2005, "6.5 mm Grendel and Alexander Arms," J. Guthrie, p. 34-37, 67-69.
 * F. Petersen's: Rifle Shooter, March/April 2006, "Cartridge Efficiency - Why case shape matters," M.L. McPherson, p. 22-24.
 * G. Shooting Times, January 2007, "Other AR Chamberings," Sidebar Article, David Fortier, p. 56.
 * H. Special Weapons, Semi-Annual #50 2007, "The Super Versatile AR," Charlie Cutshaw, p. 44-45, 80-83.
 * I. Special Weapons, Semi-Annual #50 2007, "5.56mm NATO Alternatives," Stan Crist, p. 52-59.
 * J. Shooting Times, March 2007, "Les Baer's 6.5mm Grendel AR Sets a New Standard," David Fortier, p. 26-32.
 * K. Special Weapons for Military & Police #52, Spring 2007, "BETTER-IDEA 6.5mm GRENDEL," Stan Crist
 * L. Special Weapons for Military & Police #52, Spring 2007, "New Battlefield Requirements - New Rifles and Ammo Needed," Charlie Cutshaw