Ballistics out of a 6.75″ 300 Blackout barrel

Last Saturday I had the chance to fire some handloaded ammunition out of my 6.75″ barreled 300 Blackout AR15 pistol.

Next weekend I am taking my first rifle class of 2014 and I wanted to  make sure the AR15 was ready, function test my M80-based plinking rounds, and also chronograph my defense loads. More and more of you have been asking about if a 6.75″ barreled .300 Blackout gives up too much in the velocity department, with some asking if it would be better to run a 9mm or 10mm pistol caliber carbine instead.

Please note that these results are for my AR15, with my configuration, with my handloaded rounds. Your experience may vary and this is not meant to be official load data. Handloading is always done at your own risk.

300 blackout barnes bullets tac-tx

 

Load recipe

My current defense load:

  • 110 grain solid copper “barrier blind” TAC-TX projectile from Barnes
  • .300 Blackout brass resized from 5.56 Lake City blanks
  • CCI #400 primers
  • 18.0 grains of Alliant 300-MP powder
  • Cartridge overall length: 2.234″

My test ladders were 18.0, 18.3, and 18.6 grains.

Test results

All rounds fired successfully and cycled my AR15 pistol. All recipes locked the bolt back on the last fired round.

No recipes resulted in case damage, flattened primers, or ejector swipes. The 18.6gr load resulted in some extremely minor primer distortion.

The average velocity of my “approved” load was 1860 FPS. The lowest FPS was 1847, the highest 1888. The mean was 1847. Calculated energy is 883 ft/lb.

My instructor Erik Pakieser fired some 147gr 9mm Federal HST out of his 5″ barreled Smith and Wesson M&P 9L pistol.  His velocities were between 890 and 990 FPS. Calculated energy is 259 ft/lb.

He also fired 115gr hardball at between 1148 and 1177 FPS. I imagine that 115gr HST rounds would give better performance, but by how much would just be my speculation. Calculated energy is 354 ft/lb.

An AR15 shoots a 55gr 5.56x45mm projectile at about 2200 FPS out of a 7.5″ barrel. Calculated energy would be 591 ft/lb. Furthermore, the 5.56x45mm round will not fragment as such a low velocity, which is one of the selling points of the round. These reasons are why the She-Shepherd’s SBRed AR15 in 5.56x45mm has a 11.5″ barrel.

For a back of the envelope comparison, my SBR AK47 with a 7.5″ barrel supposedly pushes a 123gr projectile at 1900 FPS, which means 986 ft/lb.

Future testing

I stopped at 18.0 grains of Alliant 300-MP powder. Someone is loading 20.0 grains of Alliant with their Barnes TAC-TX projectiles. This results in 2356 FPS out of a 16″ barrel and some primer flattening. If I never re-used this brass I might be okay with loading it hotter. I wonder what the recoil impulse is like.

Please note that these results are for my AR15, with my configuration, with my handloaded rounds. Your experience may vary and this is not meant to be official load data. Handloading is always done at your own risk.

About the Author:

Short Barrel Shepherd Short Barrel Shepherd is a regular guy and works to make Web sites and mobile apps easier for people to use. He spends his free time attending fight-focused firearm, knife, and combatives training, motorcycling, writing, and playing games. His daily carry is a Glock 19 pistol and an AR15 .300 Blackout pistol in a backpack.
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2 Comments on "Ballistics out of a 6.75″ 300 Blackout barrel"

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  1. Eric says:

    How is reliability with this setup? Im currently planning a .300 blackout PDW build, and was wondering how short I could go. How is the muzzle blast?

    • Short Barrel Shepherd Short Barrel Shepherd says:

      Every issue I had with my .300 was due to how I was making my ammunition. You may want to peruse this post I wrote (it’s about a bunch of stuff, some .300-specific, some not). You’ll also see some reader comments about their .300 experiences.

      http://shortbarrelshepherd.com/season-one-with-a-300-blackout-ar15/

      In general, no problems with reliability with my build. 100% of the problems were due to poor shoulder forming and pocket primer crimp removal, and I take full responsibility.

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