I was thinking if someone wanted a subsonic 30 caliber round and could load either single shot or a bolt action mag of intermediate (ie 223ish) length, how about using the 30 carbine but with much heavier bullets that simply wouldn't fit in the M1 Carbine mags? Wonder how big a bullet could be used effectively. The thought of a 240gr bullet in a 30 carbine shell is... um... amusing. Course that would be more like a 300 Whisper, so I doubt the 30 carbine round can propel it that fast. Course if it were designed for a bolt gun it could probably be loaded abnormally hot. Could use any 308 caliber barrel blank. Low cross sectional area would mean far lower rearward thrust than most other rounds. Suppose it could even work in a straight blowback carbine? Gears are turning here, what do you guys think?