300 AAC Blackout Subsonic Load Recipe: 190gr SMK using IMR 4198

Count IMR 4198 (as you can see in the photo it’s an extruded powder) among the options for powders that can be used for loading subsonic rounds that cycle in AR platform rifles. Recipe was 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.

Performance: This recipe successfully cycles both a carbine length upper (and gas system) with a 16″ barrel and also a 9″ pistol length upper, w/o suppressor. No bolt hold-open on either upper in my testing. No signs of overpressure or instability. The test cartridges sounded quieter than the LilGun loads I fired afterward. Chrony readings coming soon. 

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Caliber: 300 AAC Blackout (300 BLK)

Projectile: 190gr Sierra Match King HPBT

Primer: Winchester Small Rifle Primers

Powder: IMR 4198

Measure: 11.0gr

Lee Auto-Disc setting(double disc): Top Disc 0.43 | Bottom Disc 0.47

Min OAL: 2.222

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:

  • 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 (although 190gr SMK is a long projectile and as such will require a longer min OAL than the range defined in the SAAMI spec)
  • My personal Min OAL: 2.140 to accommodate projectile length

This seems like a pretty big tolerance but remember that the BLK supports a wide range of projectiles that are different lengths. Since the 190gr SMK is one of the longer BLK projectiles I set my seating die to give me the longest possible cartridge-about 2.220 and ranging up to 2.240. 

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