The original 300 Whisper was based off of the 221 Fireball case. There are folks calling the 223 necked up to 30 caliber a 300 Whisper as well. The original was designed back in the 90's by SSK Industries. It fires a 200+gr bullet at subsonic velocities or you can drop down to a 125 gr bullet at about 2400'ps, or about the same HP as a 30-30 or 7.62x39. I'm not a fan but understand a lot of folks like to play with something odd. If subsonic is the goal, you can buy subsonic ammo all day long for a 308 from various makers. It's well tuned for the 308 and twist rates. The Gov't gets theirs from TTI using a 170gr bullet. Accuracy is amazing considering. With a muffler on the end of the barrel, the TTI ammo makes zero noise. All you hear is the firing pin drop. Obviously, trajectory is an issue if you've never shot it before.