178gr A-Max Subsonic 300 AAC Blackout Load Recipe

Hornaday makes some really interesting projectiles. While the gel block above is a supersonic impact from a .308 you still get the idea that this could be a sweet BLK projectile as a sub or a super. A really thin casing and excellent expansion are the hallmarks of this projectile and should pack a punch. As if the gel block wasn’t compelling enough the loaded cartridges have a red tip that looks positively evil. Do me a favor and don’t let the Antis or Common-Sense Gun Violence Prevention Groups (sic) learn about these- they look so damn scary we may have to fight another gun control law over them.

Many of the newer Hornaday projectiles have polymer tips and come in heavier weights that can be loaded for subsonic use in the 300 Blackout. The tip helps when loading a cartridge like the 300BLK because it helps the cartridge navigate the feed ramps and chamber (see OAL notes at the end of this post). The tip also resists deformation if it has a rough ride to the chamber.


Seating projectiles to a longer (but safe) length in my own Blackout ammo has resulted in reliable feeding- please be sure to stay within the safe range of COAL (lower limit= minOAL…for upper limit stay under max OAL in SAAMI specs) when tuning your own load recipes.

The bolt cycles and locks open after the last round clears in both my 16″ rifle and a 9″ AAC pistol (after seeing how nice those AAC uppers are I need to start saving my pennies for one of my own). The test rounds fired a sub-MOA group at 50 yds.

Chrony results can be found here


A word on powder: Accurate 1680 is tough to find sometimes but the powder density matches the case capacity very nicely in the BLK. More on alternative powder options here. I use CCI primers because they are a bit faster burning than Winchester and that’s handy for subsonic loads to help ensure you maximize ignition to help cycle the action. KB ALERT: Winchester or a similar, harder primer are the only safe ones to use in a Lee Progressive loader-be sure you use the correct type of primers with a Lee Progressive system.

Reloading Press

I run this load recipe on my Lee Precision 4-hole Turret Press and load both factory BLK brass and 5.56 brass trimmed to 300BLK specs.

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Load Data

Caliber: 300 AAC Blackout (300 BLK)

Projectile: 178gr Hornaday A-Max

Primer: CCI Small Rifle Primers (no. 400)

Powder: AA1680

Measure: 10.8gr

Lee Auto-Disc setting: 0.71

Min OAL: 2.210 in.

Max OAL: see discussion below

my optimal OAL: 2.223-2.227in.


Regarding OAL I had to experiment to find a length that fed well. Here is my thought process to arrive at the OAL settings I use:

This seems like a pretty big tolerance vs any handgun OAL range. Some factory ammo I purchased didn’t feed well due to FTC (Failure to Chamber). It turned out that they were the short end of the SAAMI OAL specs. Some scouring on forums and experimentation proved that longer OAL fed better : no FTCs on any of my BLK cartridges since this change.


2 thoughts on “178gr A-Max Subsonic 300 AAC Blackout Load Recipe

  1. Pingback: Chronograph results- at long last | kR-15: info and resources for firearms enthusiasts

  2. Pingback: Subsonic Projectile Alternatives for the 300 BLK | kR-15: info and resources for firearms enthusiasts

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