By March 7, 2014 October 4th, 2016 No Comments

Ever since I first read one of Foghorn’s 300 BLK fanboy articles back in early 2012, I’ve been hooked on the caliber. 300 BLK gives you essentially the same ballistics as 7.62 x 39 out of the ubiquitous AR-15 platform with only a barrel change. Everything else in your stock 5.56 AR can stay the same and you get to use the full 30-round capacity of your 5.56 magazines (offer void in CA, MA, NY, NJ, etc., etc.). The caliber is designed to be shot both supersonic and subsonic and unlike many other gas piston guns, you don’t need to fiddle with the gas settings when you screw on a suppressor. Really, what’s not to like? Well, pricing and availability are less than stellar, but 300 BLK is a cinch to reload and if you purchase clean and pre-sized brass from Carolina Brass, you can be cranking out your own rounds by the dozen in minutes with a very rudimentary reloading rig . . .

A while back, Foghorn told us about his latest project, a 300 BLK Short Barrel Rifle that had just recently cleared paperwork. That post prompted a lively exchange on a number of topics, one of which was the performance of 5.56 versus 300 BLK in SBRs. So I fired off an email to Foghorn and suggested that he do a comparison shoot off between 5.56 and 300 BLK in both full length and SBR length barrels and compare the results. Nick thought this was a peachy idea and in the meantime he sent me a link to a comparison someone else had done with 55 grain 5.56 and 110 grain 300 BLK in different barrel lengths from 4 inches up to 24 inches. For the curious, that chart can be found here.

But Nick’s a pretty busy guy and with my recent purchase of an AAC 9” 300 BLK upper, I had all of the tools I needed to do the comparison myself. With that in mind, I set up the following test:

For the .223/5.56 mm sequence, I decided to try both 55 grain and 62 grain M855 ammo. I wanted to use M193, but didn’t have any kicking around, so instead, I used some Fiocchi 55 grain .223.


Test weapons were my 16” SIG Sauer 516 Patrol Rifle and my SIG Sauer 10” P556 pistol.


For the .300 BLK testing, I opted for Remington UMC 115 Grain 300 BLK ammo


and used both my 9-inch and 16-inch AAC 300 BLK uppers. For the curious ATF agents out there, you can relax. The 9-inch upper was mounted on a legitimate pistol lower receiver. Unfortunately, this isn’t exactly and apples-to-apples test. Ideally, the shorter barrel lengths for the two calibers should have both been either 9 inches or 10 inches, but one must work with what one has.


The test was conducted on the same day to minimize the impact of temperature and humidity differences. For each string, 20 shots were fired through my chronograph and the average velocities taken. Finally, the velocity of each load in each barrel was compared and the percent reduction calculated.

There are a couple of things to note here. First of all, remember that while the 5.56 round had to contend with a 6-inch reduction in barrel length (16 to 10 inches), the 300 BLK had an even larger reduction – 7 inches (from 16 down to 9 inches). Even with this handicap, the velocity reduction of the 300 BLK versus the 5.56 was very close on the 55 grain bullet and 6% less than with the 62 grain projectile.

In an effort to adjust for the one inch difference in barrel length between the two calibers, consider that the chart Foghorn sent me predicted a 9.35% reduction in velocity of a 110 grain 300 BLK round. This would put it relatively close to the measured 55 grain .223 reduction and much better than the 62 grain velocity reduction. Further consider that the same chart predicts an 11.7% reduction in velocity for the 110 grain test round when moving from 16 inches to 9 inches. My testing showed the reduction was actually even less – 10.11% so its fair to suggest that had I fired this round from a true 10 inch barrel, the velocity reduction percentage might very well have been even less than the 55 grain .223 reduction.

The second thing to keep in mind is that velocity tells only half of the story. To determine a round’s effectiveness, you also have to consider bullet weight. While it’s true that the 115 grain 300 BLK round that was tested had a lower velocity across the board than the two .223/5.56 rounds that were tested, the 300 BLK was also about twice as heavy. In order to compare the three rounds another metric must be used to normalize the results – Muzzle Energy.

Muzzle Energy is a measurement of the kinetic energy of a round as it leaves the barrel. Both bullet weight and bullet velocity contribute to total kinetic energy. Because muzzle energy takes both weight and velocity into account, it allows us to calculate a single value for a particular load and enables comparisons between loads of different weights and velocities. That said, muzzle energy is not the one metric to rule them all because as soon as a bullet leaves the muzzle, other factors such as gravity and aerodynamic drag start to impact the bullet’s flight. Since the goal here is to draw a comparison and because I’m really bad at advanced mathematics, I’m not going to try and create a high level model that takes multiple factors into account. Instead, I’m going to calculate muzzle energy, compare the three rounds, and then go get a beer.

Since I’m lazy, I used Verne Trester’s muzzle energy calculator to figure out the energy of each of the tested rounds. Here are the results:

So, what were my conclusions? Well, first of all, a 115 grain 300 BLK round has more muzzle energy inch for inch than anything in the 5.56 or .223 range. No real surprise there. What’s more interesting is what happens when you cut the barrel down. The 300 BLK’s muzzle energy out of a nine inch barrel was within 100 fps of the 62 grain 5.56 coming out of a 16 inch barrel and when compared to the 62 grain 5.56 out of the 10 inch barrel, it was no contest. The 55 grain .223 bullet isn’t even in the game when compared to the performance of the 300 BLK.

In an AR-15 platform, there really is nothing that the 5.56 round can do that the 300 BLK can’t do better. Sure, for very long distances, the 5.56 might have a slight advantage, but with his 9 inch barrel, Foghorn was able to ring the 500 yard gong with supersonic ammo and could consistently hit the 250 yard gong shooting the slower subsonic ammo. We already know from the calculations above that once you cut the barrel down on a 5.56 round, you really impact its performance, so to keep up with the 9 inch 300 BLK SBR, you would need to retain your 16 inch barrel for 5.56. Now, for real distance work there are still better AR-15 calibers – 6.8 SPC for one. But then again, I have my .308 LWRC REPR, my 300 Win Mag Bolt Rifle, and my .338 Lapua for distance shooting, all of which will be superior to just about anything in the AR-15 platform.

A SBR is not likely to be something that I use for real distance work – it is a close-in gun. Based upon the results of my testing, assuming that you can overcome the cost and lack of availability problem of 300 BLK (and you either own or plan to get a suppressor), the 300 BLK is my choice for SBR caliber.

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