While the majority of rifles built on the AR-15 platform still use the ubiquitous .223 Remington and 5.56x45mm NATO cartridges, hunters and shooters do have a couple of other options if they want to use a different cartridge in their AR. Today, we’ll discuss two in particular: the 6.5 Grendel and the 6.8 Remington Special Purpose Cartridge.

Gun designer Bill Alexander successfully developed the heavy hitting .50 Beowulf cartridge in the early 2000s, which is one of the largest caliber cartridges that will function in an AR-15. Fresh off that success, he helped design the new medium bore cartridge for the AR-15 that we now know as the 6.5 Grendel a few years later.

Built by modifying a 6.5mm PPC case, the 6.5 Grendel functions very well in an AR-15 and is known for outstanding accuracy and mild recoil. While cartridges must fit within relatively strict dimensional constraints to function in an AR-15, the 6.5 Grendel uses a pretty efficient design and shoots aerodynamic bullets that retain energy, resist wind drift, and minimize bullet drop very well.

The 6.5 Grendel also offers a significant step up in power compared to the 5.56 NATO cartridge. Exact figures vary depending on barrel length and the specific load used. However, Hornady’s load featuring a 123gr SST at 2,580fps is a good example of typical 6.5 Grendel ballistics with modern factory ammo. With just over 1,800 foot-pounds of energy at the muzzle, that load has approximately 40% more muzzle energy than the M855 5.56x45mm NATO load commonly used by the various branches of the United States military.

Around the same time Alexander was working on the 6.5 Grendel, the United States Army Marksmanship unit was working in tandem with the Remington Arms Company to accomplish a similar goal of building a more powerful alternative to the .223/5.56 cartridge that would still function in the M-16/M-4.

Essentially, they shortened and necked down a .30 Remington case to shoot a 6.8mm bullet. Officially designated the 6.8mm Remington Special Purpose Cartridge (also known as the 6.8mm Remington SPC or 6.8 SPC), the new cartridge is another bigger bore alternative to the .223 Remington that functions reliably in an AR platform.

Like the 6.5 Grendel, the 6.8 SPC is also considerably more powerful than the 5.56x45mm NATO cartridge. Hornady also manufactures a load for that cartridge using their SST bullet. This particular load pushes a 120gr SST at 2,460fps using a 16-inch barrel. That’s not quite as powerful as the 6.5 Grendel, but it still produces over 1,600 foot-pounds of energy at the muzzle, which is nearly 30% more than the M855.

Keep in mind that the 6.8 SPC produces those ballistics with a relatively short 16” barrel as well. While the 6.8 SPC does not use extremely aerodynamic bullets like the 6.5 Grendel, the Remington cartridge actually functions remarkably well with shorter barrel lengths without nearly as much of a drop off in performance as many other comparable cartridges.