The BLK was designed to accommodate both supersonic and subsonic ammo without any tuning or performance headaches. The subsonic loads are intriguing: who wouldn’t want to send a 220gr Sierra HPBT for a big, heavy projectile to create high performance at subsonic fps. That’s a mere 3.4 times heavier than common 62gr 5.56 ammo. 5.56 ‘heavy’ projectiles weigh in at 75-80gr. The BLK was designed to run suppressed too- so if real quiet os your bag and you I enjoy some closer range shooting (effective hunting range is about 100m).
This is the recipe I use for supersonic loads for my Model 1 300 Blackout Carbine. About 50 rounds of this recipe fired through my 16″ rifle at distances up to 200 yards to punch paper and plink. I’m using 220gr Sierra Match HPBT for the projectiles and AA1680 for powder. The bolt locks open after the last round clears.
Accurate 1680 is tough to find sometimes but the powder density matches the case capacity very nicely in the BLK. rI 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.
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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- The standard AR-15 mag accepts cartridges up to 2.260 OAL
- The SAAMI Specs show an acceptable range of 1.78-2.260 OAL for the 300 BLK
- Min OAL: 2.140 to accommodate projectile length