.300 BLK Ammo Question re: super/subsonic

[Tommygunn]

I recently purchased a Sig MCX "Rattler" AR Pistol in .300 Blackout. The weapon has an adjustable valve setting on the gas piston mechanism to select for supersonic or subsonic ammo.
My question is, that while some brands appear to state on the box if it's super or sub sonic, other brands do not. If it is not marked, but the muzzle velocity is (it appears atleast that is consistently marked) what is the cutoff point between supersonic ammo and subsonic ammo?

 

[badkarmamib]

Speed of sound is ~1125 FPS. Anything below that is subsonic.

 

[Tommygunn]

Thanks, Badkarmamib.

 

[jonnyc]

Usually determined by bullet weight. Less than 165 will usually be supersonic, above 180 usually subsonic. That buffer zone could be either, depending on loading.

 

[patmccoy]

Speed of sound is ~1125 FPS.

True at about 65 degrees F. It is about 1150fps at 90 degrees F. Speed of sound decreases with less dense air (lower temperatures).

 

[luzyfuerza]

Speed of sound is ~1125 FPS. Anything below that is subsonic.

The "cutoff" isn't quite that binary. Velocities that are close to the local speed of sound (both above and below) are referred to as transonic. Bodies travelling at transonic speeds experience conditions that are neither supersonic nor subsonic...kind of a compromise between or a mix of the two.

 

[badkarmamib]

Okay, speed of sound depends on temperature, humidity, altitude, and a number of other factors. Advertised muzzle velocity is with their test barrel, on their test day, with their test conditions. So, the correct answer is, "It depends". The safe answer is, "Use the absolute heaviest, slowest load you can find." The practical answer is, "Stay under 1100 FPS and you should be okay with the majority of loads, especially in a short barrel like a Rattler."