When I started researching recipes for 300 BLK subsonic loads the recommended powder was Accurate 1680. The Interweb phenomenon took hold and now there is a false impression that 1680 is the only powder that works for subsonic AAC Blackout loads.
Slowly the truth is getting out there- and I can now personally confirm success with LilGun as an alternative to 1680 for subsonic BLK loading. Also LilGun requires less charge per round than 1680– so it’s even more efficient– it’s my new preferred powder. More alternatives here– I plan to work on IMR options next.
Recipe created and tuned using a Lee Precision 4-hole Turret Press with double-disc powder throw and load both factory BLK brass and 5.56 brass trimmed to 300BLK specs. I recommend using LC brass when possible as I find its the most consistent. I still use CCI #400 primers, although you should note that these primers are a little soft so CCI #41 primers are best.
Performance:This recipe successfully cycles both a carbine length upper (and gas system) with a 16″ barrel and also a 9″ pistol length upper, no suppressor. No bolt hold-open on either upper, but that could be the PMAGs I used. No signs of overpressure or instability. Chrony readings coming soon are here and I had no problems dinging a 100yd target with these without hold-over.
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Caliber: 300 AAC Blackout (300 BLK)
Projectile: 220gr Sierra Match HPBT
Primer: CCI Small Rifle Primers (no. 400)
Powder: Hodgdon LilGun
Lee Auto-Disc setting(double disc): Top Disc 0.30 | Bottom Disc 0.30
Min OAL: 2.140
Max OAL: see discussion below
Regarding OAL I had to experiment to find a length that fed well. Here is my thought process to arrive at the OAL I set my dies to:
This seems like a pretty big tolerance. I set my seating die to give me the longest possible cartridge-about 2.220 and ranging up to 2.240. 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.
The beauty of using these high mass projectiles in a modern sporting cartridge is that they pack a serious punch and don’t use much powder– a nice bonus when supplies are scarce or you’re on a budget.