300 BLK suppressed AR Pistol subsonic reloading

[mnoirot64]

Hey Guys & Gals! It's been awhile since I posted. I am an experienced reloader having rolled my own ammo for over 20 years. I have loaded 300 BLK subsonic/supersonic for carbine length ARs for five years - with very good results.

Last May I purchased a SIG Sauer Rattler with an 8" barrel. I have only shot the pistol a few times as I was waiting for my tax stamp for a suppressor. After nine months I finally picked up my suppressor last Wednesday. Taking it to the range with 208gr HPBT reloads, which worked for my 16" barreled AR, provided quiet shots, but they are obviously too under-powered to cycle the pistol. I have tons of new reloading guides from Hodgdon, Speer, Hornady and Lyman. Unfortunately, none of the manuals provide a starting point for an 8" AR 300 BLK. Where does one even begin?

The carbine length subsonic recipe for the AR is:

208gr Hornady BTHP
8.1gr Vihta Vuori N110
Remington 5 1/2 small rifle primer
New Starline brass
COAL 2.250"

I also have Hodgdon H110 which works well for the carbine. Accurate 1680 seems to be the most popular powder, but burn rates for the H110/N110 seem similar to 1680.

Has anyone worked up subsonic loads for 8" 300 BLK with either of the powders I have?

Mike
Clarksville, TN

[Sharkbite]

No experience with subs with your powders. I use 1680 and 208gn Amax’s in my 9” AAC SBR.
I do use H110, but only for supers with 110-125gn bullets.

Its less about burn rates then gas volume.

[mnoirot64]

I guess I may need to buy some 1680. It seems to be readily available.

[FrankenMauser]

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With H110, you'll find that nearly every published load for subs falls between 8.5 and 13.5 gr; with many not specifying start or max, but 180+ gr bullets just giving a single load of 9.2-9.5 gr. (Outliers are typically much higher and eyebrow-raising.)
I don't even bother checking load data any more for 180+ gr bullets.
I load 9.5 gr and test. If it cracks, I load a few at 9.2 and maybe 9.0 gr.
Works every time.

My barrel is 9.5", pistol gas, running on a pistol lower w/ standard carbine buffer and spring, or SBR lower with standard rifle buffer and spring. If you are running a heavy buffer, special spring, or super high tech gas system component, it/they should be suspected of causing the failure to function.

I always test a new load unsuppressed, to make sure the bullets are not tumbling or yawing excessively, to prevent damage to the can. I have not yet found a 'final' load that did not cycle unsuppressed.


If you want to turn to 1680, consider Shooter's World Blackout or Shooter's World SBR SOCOM. They are nearly identical and are derived from the base propellant for the older formulation of 1680. But they are cheaper and don't seem to suffer availability issues as much as 1680.
If you choose SBR SOCOM, make sure you have data before ordering. It is not as easy to find as data for 1680 or Blackout.

[mnoirot64]

The advice is much appreciated! Thank you.