I gave this some actual thought earlier, and came up with a solution that would be better for me. A 300 BO pistol in 8 or 9 inches, would be using a Stag LH upper and BCG (I'm a southpaw, pistol gas sends brass to my face past the deflector on a RH), subsonic only, with a LT folder and a short featureless pistol buffer tube with a Phase 5 tube cover.

Short, foldable for transport in a discrete backpack, low round count SHTF only other than a zero because I don't want to invest or reload for 300 BO as it's a niche to me and this thread fits the bill for it's niche role, and not as traumatically loud as 5.56 with a can on it.