6.5mm Grendel

The 6.5mm Grendel (6.5×39mm) is an intermediate cartridge designed by Arne Brennan, Bill Alexander and Janne Pohjoispää as a low-recoil, high-accuracy cartridge specifically for the AR-15 platform at medium/long range (200–800 yard). It is an improved variation of the 6.5mm PPC. Since its introduction, it has proven to be a versatile design and is now expanding out into other firearms including bolt-action rifles and the Kalashnikov system.
The name "6.5mm Grendel" was a trademark owned by Alexander Arms until it was legally released to allow the cartridge to become SAAMI standardized.
Development and history.
The 6.5mm Grendel design goal was to create an effective 200–800 yard STANAG magazine-length cartridge for the AR-15 that surpassed the performance of the native 5.56mm NATO/.223 Remington cartridge. Constrained by the dimension of the magazines, the Grendel designers decided to use a shorter, larger diameter case for higher powder volume while allowing space for long, streamlined, high-ballistic coefficient (BC) bullets. Firing factory-loaded ammunition with bullets ranging from , its muzzle velocity varies from with 129- and bullets to with bullets (similar in velocity to a 5.56 mm round). Depending on their case material and bullet, 6.5 Grendel cartridges weigh .
The case head diameter of the Grendel is the same as that of the .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 capacity of standard size M16/AR-15 magazines. A Grendel magazine with the same dimensions as a 30-round STANAG magazine will hold 26 rounds of 6.5mm Grendel ammunition.
Performance.
Proponents assert that the Grendel is a 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. 
For example, the 6.5 Grendel has more energy and better armor penetration at 1,000 meters than the larger and heavier M80 7.62 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. 
External ballistics.
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 (6.1 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.
Army and police uses.
Serbia is in process of adopting a rifle made by Zastava Arms in 6.5 mm Grendel caliber as main armament for its armed forces. The rifle, designated M17, is a derivative of the previous-issue M70 rifle. 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 Užice Serbia will be tested for use with these rifles.