Yes? No?

Is there such a thing?

The amount of pressure to make a semi function is not created by subsonic ammo. Most won't even come close to ejecting the round, if even extracting it partially.

Look at the 300 Whisper. for it to function, the typical set up has the gas port located at the pistol length, with a 10-11"+ barrel to maximize dwell time of the pressure.

For .223 or .308 sub to cycle, the gun would have to be purpose built for running subs, and then usefulness with standard ammo would be questionable.

IMO, .223 subsonic is useless, and .308 is fun, but not practical. It's diameter is small compared to subsonic pistol rounds, and available bullets don't perform well at the low velocity. They penetrate, but don't expand much.

Quoted: The amount of pressure to make a semi function is not created by subsonic ammo. Most won't even come close to ejecting the round, if even extracting it partially. Look at the 300 Whisper. for it to function, the typical set up has the gas port located at the pistol length, with a 10-11"+ barrel to maximize dwell time of the pressure. For .223 or .308 sub to cycle, the gun would have to be purpose built for running subs, and then usefulness with standard ammo would be questionable. IMO, .223 subsonic is useless, and .308 is fun, but not practical. It's diameter is small compared to subsonic pistol rounds, and available bullets don't perform well at the low velocity. They penetrate, but don't expand much.

+1.  Good post.

Quoted: The amount of pressure to make a semi function is not created by subsonic ammo.

So how do you explain cycling subsonic .223 and .308 ammo?

Ive never seen any that did....

Quoted: Quoted: The amount of pressure to make a semi function is not created by subsonic ammo. So how do you explain cycling subsonic .223 and .308 ammo?

They don't cycle.  All the subsonic I've ever shot in a centerfire rifle made the gun single shot.