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.
Flash can / flash forward device,
Is a great addition to any SBR or AR pistol.
First time I went to an indoor range there were 4 out of 6 Lanes being used.
I did not have Flash Forward device on my AR pistol .223. Just a VG6 muzzle brake .
it was a huge concussion in my Lane,
And after the first two 10 round mags everybody left.
Not long after I went to an outdoor range, and the guy in the lane next to me moved down two more spots.
I purchased the Flash Forward device and it was a game changer.
It definitely directs the Flash and concussion forward rather than not having one.
My particular Flash Forward device has a mounting ring that goes on behind the muzzle brake and the Flash can screws on over the muzzle brake.
https://postimg.cc/SJtMb7c8