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Range Report on 300 AAC Blackout on AR-15 Platform
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Old 09-28-2017, 05:00 PM
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Default Range Report on 300 AAC Blackout on AR-15 Platform

Some may recall that I have been 'fiddling' with all sorts of subsonic cartridges, loads, firearms, etc. for the past several months. (Actually, considering when I started using the .22 RF 60-grain Aguila SSS, it's been years.) When dealing with subsonic projectiles, it's all about delivered energy. Impact velocities are by definition less than the speed of sound. (~1130 f/s) Since you can't go faster than that, if you want to deliver more energy to the target you have to increase the weight of the projectile. Staying within a caliber, increasing weight means increasing the length of the projectile. Increasing the length AND decreasing the velocity of the projectile means addressing issues associated with gyroscopic stability. So...

My efforts in this vein have included:
1) Shooting the Aguila 60-grain SSS cartridge in my Ruger 10/22. Where I live, the standard 1:16 twist rate .22 RF barrel will not stabilize that bullet. I purchased a few aftermarket 1:9 twist barrels, and got the SSS to shoot "pretty good". However, wiht a MV of only 850 from the 16.5" 10/22 bbl, that 60 grain bullet only has about 85 ft-lbs of energy at 50 yd. That "works", but it's literally 'weak'.
2) One rifle I have (Stevens USGI 10X), shoots the Aguila at an average MV of ~950 f/s, but it has the slow twist and won't gyrosopically stabilize that long bullet. Wanting both more MV and a little more precision, I decided to make a bolt action rifle in hopes of getting both. I bought a CZ-450. That rifle has a barrel-to-receiver mount that allows quick-change capabilities of multiple barrels. The factory barrel has the 1:16 twist rate that won't stabilize the SSS bullet where I live, but... I bought some 1:9 twist, .22 RF barrel LINERS, and made a barrel for the CZ-455. Results with the SSS and other subsonic ammo were satisfactory precision-wise, but I didn't get the higher MV I was hoping for.
3) I also tried to get the .22 Hornet cartridge, loaded to subsonic velocities with "heavy" .22 CF bullets to "work". In spite of trying several different 'tactics', I never really did get a satisfactory subsonic load. Which brought me to...
4) The .300 AAC Blackout cartridge. One which was designed for the AR-15 platform AND designed for subsonic application. One day while perusing the Green Mountain Barrel site, I saw that they had PRE-CHAMBERED, barrel-extension-installed, muzzle threaded, AR-15 barrels chambered in 300 Blkout. $95 delivered to my door. I bought one. The gas port was NOT drilled so the user could use whatever length gas system they wanted. That was even better for me, because I didn't want semi-auto operation, as I was going to configure the rifle for manual operation.

I have spent the past 6 weeks or so trying to get the thing to do what I wanted! Even though the cartridge was designed for subsonic application, it still suffers to some degree the same problems other cartridges do: The conflict between the small charges (low case capacity use), with heavy bullets. This issue of low case capacity occupied by the powder leads to inconsistent MVs due to powder position sensitivity causing inconsistent ignition and burning characteristics. This problem is mitigated to SOME degree by increasing bullet length taking up more of the combustion chamber. However, when you reduce the combustion chamber, you reduce the powder required for the same pressure/velocity. Still, there was some 'good' to be realized there.

My first tries were with bullets I had on hand. I had three potential candidates:
1) The Hornady ".303 British" bullets SUPPOSEDLY weighing 174 grains, but actually weighing only 170.
2) The Sierra 165 grain .30 caliber GameKing, and
3) Some bullets that years ago I pulled from some 7.62x54R Russian ammo weighing 180 grains.

Those initial bullets and loads were "uninspiring", so I bought some bullets specificially for this effort. First was a 180-grain, copper-washed bullet made by Ranier specifically for the 300 Blkout. Pretty good looking bullet:

Not horribly priced, and shot pretty good from my rifle with I4227


and

BUT... The charge only occupied about 60% of the available case capacity, and the powder was sensitive to "position". So if I remembered to raise the rifle's muzzle to the vertical and slowly lower to the horizontal shooting position, the groups were 'fine'. But if I didn't, the MVs were erratic and of course the precision went 'south'.

The same was true for the 170 grain round-nosed Hornady and Sierra bullets I bought. They shot OK, but the charge was position-sensitive.


Since I want (at some point in the future) to shoot these bullets through a suppressor, I couldn't take up "space" with anything that would be 'caught' by the suppressor. However, I did find some "buffer" for shotgun pellets that was supposed to work as a "buffer" in CF cartridges. I bought some and tried it with the Sierra and Hornady 170s. Horrible result. I have no idea where shot #1 went!


I thought I would try to decrease the case capacity by going to heavier bullets, so I bought some Hornady and Sierra round-nosed 220s. (I'm buying round-nosed bullets for two reasons, 1) they're shorter for their weight, and 2) they will "hit harder".) Preliminary shots with these bullets were DISASTERS!








About to start pulling my hair out, I went looking on the internet and discovered that Hodgdon had come out with a powder just for the 300 Blackout - CFE BLK. Unfortunately, the reviews were mixed, and most of the ones that appeared objective (not shills for Hodgdon), were negative. I pressed on with the I4227. TO NO AVAIL. So, pretty much at wits end, I decided to "start over" with the CFE Blkout powder.

I remeasured the chamber length, and got a preliminary load from YouTube. (I had learned through the above exercises that the gun WANTS the bullet ON the lands!) I forgot to mention above that I had also purchased a neck-only resizing die and a "Factory Crimp" die, and was using them exclusively. I had been using a pretty stiff crimp. For the CFE Blkout loads, I increased the OAL by 0.100" and reduced the crimp to "breathed on". Here is the first set of 5 shots each with the Hornady and Sierra 220s:


THAT is what I was after! I made no attempt to 'accommodate' the powder's position. These particular loads were just supersonic, and have the slightest 'crack' to them, so I will lower the charge by 0.1 grains and give them another whirl. Barring some disaster associated with that reduction in charge, I think I have found my "heavy-hitting" subsonic rifle, cartridge, and load. A bullet weighing 220 grains and doing 1083 f/s at the muzzle, will be doing 1050 at 50 yds and have 538 ft-lbs of energy. That's just a bit more than the SSS's 85 ft-lbs.

Paul
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Last edited by gitano; 09-29-2017 at 10:53 AM..
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300 aac blkout, range report, subsonic


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