10.5" AR15, .223, with a "flash can" instead of a flash suppressor. That directs the flash/sound/concussion forward of the shooter instead of sideways around the flash suppressor/muzzle break.
I don't have a .300 BO pistol. I do have .300 BO carbine. For the cost of the heavy match bullets (I reload) I can buy 4 times as many 55 grain bullets for the .223 vs. the 220 grain HPBT in .30 caliber.
I've never shot a 220 grain bullet out of the .300 BO carbine. I've shot 150 grain FMJBT and soft point boat tails and the darn thing kicks more than my GII .308 with the same bullets moving a lot faster. It's got an H2 buffer in it and still kicks harder than anything but the M1A or M1 Garand. The BO even has a carbine length gas system, not a pistol length gas system, so port pressure will be lower on the carbine than on the pistols - and it still kicks hard. Even with the faster burning powders used in the BO, the recoil is uncomfortable. I'd rather shoot the M1A or the M1 Garand.
As they say, your mileage may vary.
My youngest son has a .300 BO pistol (AR type). His only comment after shooting it to get it sighted in was, "Little sucker kicks pretty good." He hasn't mentioned taking it shooting since.