Comparing ammo cartridges is always exciting as you can often learn something new and easily find which is better for a specific task or scenario. Whether you are a hunter, shooter or just a recreative plinker, overviews like ours should help you and reveal each cartridge’s basic data and capability.
As a general rule, when comparing any two cartridges, you can find almost everyone some similarities and differences to a greater or lesser degree. That is no exception for cartridges compatible with an AR-15-style rifle.
|Cartridge||6.5 Grendel||300 Blackout|
|Bullet||90 to 130 gr (6.0 to 8.0 g)||90 to 130 gr (7.2 to 12.96 g)|
|Bullet diameter||0.264” (6.7mm)||0.308 (7.62mm)|
|Case length||1.52” (38.6mm)||1.368” (34.7mm)|
|Maximum overall length||2.26” (57.4mm)||2.26” (57.4mm)|
|Rim diameter||0.44” (11.1mm)||0.378” (9.6mm)|
|Max Pressure (SAAMI)||52,000 psi||55,000 psi|
|Muzzle Velocity||123 gr/ 2,580 fps||125 gr/ 2,250 fps|
|Muzzle Energy||123 gr/ 1,818 ft-lbs.||125 gr/ 1,404 ft-lbs.|
|400yds Bullet Drop & Energy||-37.5” 901 ft-lbs.||-72.5” 379 ft-lbs.|
|Powder load||26 gr||19.5 gr|
|Rifle Weight||6.0 lbs.||6.7 lbs.|
|Free recoil energy||9.12 ft-lbs.||6.16 ft-lbs.|
|Case capacity||35.0 gr H2O||20.2 gr H2O|
Without a doubt, the AR-15 platform is currently the most versatile rifle in existence, but its .22 chambering somewhat limited A.R.’s performance at a longer distance or/and at the larger game.
From the early days, many AR-15 users were trying to improve the ballistics of the platform, looking for a more powerful alternative to the original 5.56/.223 Remington chambering.
Today, we have more than 50 caliber choices for A.R. style rifles ranging from .22 L.R. to .50 caliber heavy weights.
In this overview, we will research cartridges designed to fit into an AR-15 mag and be compatible with original A.R. components as much as possible. Usually, the military was the initiator for developing these calibers ordering specific features.
Since the basic specification for the new cartridges was to fit in the standard AR-15 action and NATO STANAG magazines, building an AR-15 rifle in an alternative caliber is pretty straightforward as it will mainly require a new upper for your rifle. But since the upper receivers can be pretty costly, there are some options for an AR-15 project that may require only barrel changing, whereas others will require a new barrel, gas system, bolt and magazines to operate.
6.5 Grendel (6.5x45mm)
Even though 6.5mm cartridges have recorded more than one century of existence, they weren’t always favorites of American shooters. The .26-caliber rounds were undervalued for a long time in the USA, and it was until the breakthrough of new cartridges like the 6.5 Creedmoor, .260 Remington or 26 Nosler.
One of the best alternatives to the well-known .223 Remington or 5.56x45mm rounds, without doubt, is a 6.5 Grendel. Introduced in 2003, the 6.5 Grendel is the offspring of Bill Alexander, founder of Alexander Arms. This American firearms company is also credited for developing the big bore .50 Beowulf, a cartridge with serious stopping power that can penetrate cinder block walls or engine blocks with relative ease.
If you are an even better-informed shooter, with just a glance at Grendel’s metric dimensions – 6.5x39mm, you will assume the way it was born – by narrowing down the 7.62 x 39mm case to 6.5mm. But, of course, that’s just a very simplified version.
The 6.5 Grendel is designed to offer similar or better 7.62×39 Kalashnikov performance in AR-15 platform rifles. Not surprisingly, the Grendel is a far cousin of the 7.62×39, based on the .220 Russian cartridge, designed to accept 6.5mm (.264) bullets known for their flat trajectory and long-range accuracy.
Practically the 6.5mm Grendel demonstrates pairing Eastern bloc military M43 (7.62 x 39mm) design with modern Western 6.5mm PPC competition rounds.
While some gun enthusiasts place Grendel between the 5.56 x 45mm NATO and the 7.62 x 39mm Kalashnikov, it excels both of these service rounds in velocity and ballistic performance.
The 6.5 Grendel is a mid-power .26 caliber cartridge loaded with 0.264” (6.71 mm) bullets available in 90 to 130 grains weight range, with a 123-grain bullet being the most common.
The 6.5 Grendel performs better at a distance and offers at least 40% more power than the 5.56. As a highly accurate cartridge superior to the .223 cartridge at engaging targets up to and over 500 yards, the 6.5 Grendel was also adopted by bench rest shooters.
The 6.5 Grendel has a longer case of 1.52″ compared to 1.368″ for .300 BLK, but both have the same 2.26″ overall length, as both cartridges are constrained by standard AR-15 magazine restriction.
Compared to Blackout, the Grendel uses longer bullets with a higher ballistic coefficient. It means the 6.5 Grendel is shooting flatter with more extended supersonic flight. The skinny 6.5 projectiles are much less susceptible to wind drift than the wider .30 caliber Blackout bullets.
From the muzzle, the 6.5 Grendel offers higher velocity and greater kinetic energy than the Blackout from shorter, 10″ barrels. On the other side, the .300 AAC Blackout was designed to be shot from shorter, 10.5″ barrels of military SBRs.
The .300 Blk ammo is loaded into a wider case with the same 5.56 base diameter of the case, like the base diameter of the Grendel (0.378″ versus the 0.439″.)
That gives the 6.5 Grendel more overall case capacity while keeping the case a bit slimmer and better adapted for using more slender bullets that are pretty effective at longer ranges.
.300 AAC Blackout (.300 BLK) 7.62x35mm
Based on the J.D. Jones .300 Whisper round and reloaders’ experience, after a couple of years of development, the .300 Blk cartridge was finally perfected by a joint effort of Advanced Armament Corporation (AAC) and Remington.
The .300 AAC Blackout (.300 BLK) was introduced to the shooting public in 2010 and was approved by SAAMI in 2011.
In comparison to other alternative cartridges, the .300 AAC Blackout is the most cost-effective round with adequate stopping power. When considering compatibility with an AR-15 platform, the .300 Blackout will only require a barrel change, making it the cheapest of the alternative AR-15 calibers. In other words, you can keep the upper and lower with bolt carrier groups, buffer and trigger assembly, and of course, you can use standard AR-15 magazines.
In terms of bullet weights, the .300 BLK is a dual-purpose cartridge as it can fire almost any projectile with a .308-inch diameter, including everything from light pistol bullets to quite heavy match bullets and cast bullets.
For supersonic use, the .300 AAC Blackout can be loaded with very light for caliber projectiles ranging from 78-grains to 135-grains.
On the other side of the range, the .300 BLK can shoot very heavy .308 bullets. As the .300 Blk was designed to replace the suppressed 9mm sub-machine gun MP5SD, the Blackout rifles are intended for subsonic use and can be loaded with massive 190 to 240-grains bullets.
While the Blackout will work great with everything projectiles mentioned above and any other .308-inch bullet in between, the most common .300 AAC Blackout bullet weights are 110, 125, and 220-grains.
Although initially developed for military tactical use, offering a wide variance of bullet weights, the .300 BLK cartridge is a flexible choice for hunters as it can be suitable for taking down anything from varmints to big hogs.
For muzzle velocity, a supersonic .300 Blackout round with a 110-grain bullet will exit the barrel at 2,250 fps compared to 1,000 fps for a subsonic 220-grain bullet. Using similar-weight bullets but with a lower Ballistic Coefficient, the .300 Blackout has 40 percent less initial bullet energy and 400 fps less velocity than the 6.5 Grendel.
Though both cartridges have proved advantageous for military tactical use in the A.R. platform, the Blackout was originally purpose-built with short-barreled AR-15 weapons (10 inches). In contrast, the 6.5 Grendel was tailor-made to work best with a minimum barrel length of 20 inches.
Comparing the ordinary .300 Blackout (125 grain) load to Grendel (123 grain) common loads, the Blackout is a slower cartridge starting out with less energy than the 6.5 Grendel. When translated to hunting fields, the .300 Blackout works well enough on deer and hogs at 100-yards, while the 6.5 Grendel packs plenty of power for clean kills on similarly sized game to 300-400 yards.
Both cartridges can be efficient using suppressors, but with heavy, subsonic bullets, the .300 BLK is much quieter with a suppressor than the 6.5 Grendel.
The modularity of the A.R. platform offers you massive potential beyond your standard 5.56 AR-15. Using a standard multi-cal lower receiver, you can build many A.R. projects with minimal alterations.
The 6.5 Grendel will increase and maximize the performance of the common 5.56mm Black rifle as it stomps over the 5.56 and .300 BLK because it has a flatter trajectory and stays longer until it drops past 600 yards.
Compared to the more universal Grendel, the .300 BLK fall into the realm of specialty cartridges designed for CQB and suppressed fire. But Blackout can compensate for its narrow niche since this .30-caliber round is available in a wide spectrum of bullet weights, designs and performances.
With more than 50 years experience in the field and the testing lab, author L.P. Brezny is one of today’s most recognized shotgun experts and authors. He is a contributor to dozens of firearms publications, such as Wildfowl, Shotgun Sports, and Varmint Hunters, and he is a regular columnist in the Gun Digest annual as well as AmmoLand News.