300 Blackout reload project #1.....

by Gunner , St Louis, Saturday, July 06, 2024, 15:12 (63 days ago)

Loading for the 300 Blackout……150gr SP with H110

My first major reloading project was a 150 gr soft point using H110 powder, as most sources suggested the combo works best in the 300 Blackout, my case a 10.3” Ballistic Advantage barreled AR with a 1 in 7 twist.

To get the most out of the rifle I removed the 5x prism sight and installed a Vortex 4-12x Diamondback scope and cantilever mount. After checking a half dozen manuals and major web sites I choose 6 loads to chronograph and accuracy test.

Load data for the following is grains, average velocity for 5 shots and spread for that load

15.0 grs 1613 fps 99 spd
15.5 grs 1644 fps 47 spd
16.0 grs 1709 fps 67 spd
16.5 grs 1787 fps 92 spd
17.0 grs 1810 fps 52 spd
17.5 grs 1893 fps 76 spd

For reference Ammo Inc 150 gr TMC load chronographs 1699 fps & 85 spd thru this rifle.

For the accuracy testing I used a bunny ear bag on a steel three leg rest under the forearm and cylinder bag under the stock and fired 5 shot groups at 50 yards with the scope set to 10x. Two factors in using the 50 yard mark, first I wanted to eliminate as many outside factors as possible including me jerking the crap out of the trigger and ruining a group, this is just the starting point to see which load preforms best and then tweak it, if needed. Secondly most experienced shooters recommend 50 yards as sight in with the 150 gr bullet for a 50/225 trajectory similar to the 5.56 ammo 25/300 yards. The following pictures are of the targets shot during the testing, the number above the large circle is the grain charge for the group/load.

First thing that jumps out at you is the first round is hitting away from the rest of the impacts, I know cold bore first shot etc etc but is it??? As the charge increases the first shot doesn’t stray as much and starts making a normal group but then at a point the first shot starts straying again and the group goes vertical. Too much pressure on the bullet causing the stringing??? Velocity more than the bullet can handle to stabilize? Bryan Litz could probably answer that better than I can. I fired several rounds thru the rifle to get the scope on target, let it cool then I shot one side of the target, reloaded the mag, gave the barrel a minute to cool and shot the right side group, walked down, changed the target and repeated two more times. The rifle nor barrel ever got more than slightly warm.

The best group/ load was the 16.5 grain, also wonder if I tried a little too hard after seeing the first three shots make that itty bitty cluster and got two little groups. The overall answer is the 16.5gr load is the choice for further testing and stretching the range out to 100 yards for grouping.

Testing notes-

Scope at 10x……I have found most scopes struggle with clarity when set at max power and you get a better image with it turned down a little bit.

Chronographing loads……consistence is key, between shots use the same motion with the firearm, in this case holding the rifle up or down between shots gave different reading. I use a low ready hold of the firearm as a starting and ending point. Also aiming point over the sensors is critical, I applied one inch target dots to each sky screen rod in the same location to use as a aiming/hold position.

Gunner

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