[stagpanther]

I built an AR pistol in 300 BO and put an 7.5" barrel on it for a number of reasons. First, it's compact and easily carries/stows in tight places. My state is a constitutional carry state and has a capacity restriction for semi-autos of 5 cartridges while hunting for anything with a barrel over 8", so I put a 7.5" barrel on. With a conventional flash hider it's pretty loud--but not unreasonably so--meaning I have lots of other guns that are just as loud or louder and I have no problems firing it for extended sessions (with hearing protection). Inside 75 yards the 300BO is going to be very bad news for whatever it hits with some of my supersonic handloads.

I'm also a huge fan of 9mm AR's, though I haven't built anything with a barrel under 12." They are very easy to make, the very definition of limitless cheap ammo available. The most fun you can have with an AR for plinking IMO is a 9mm. In your case, that would also have the advantage of being the same cartridge as your handgun.

The difference between the two--IMO--is the over-all reliability of the 300 BO is superior to the 9mm, the 300 BO fires rifle bullets and the 9mm fires pistol bullets which tend to be a bit more "sticky" on chambering depending upon the bullet profile.