Lehigh Defense makes some "Controlled Fragmenting" rounds, but if I recall correctly, they are fairly expensive.
Make sure you are rock solid on the laws as well concerning an AR Pistol with that new brace. I've read on numerous "reputable" sites that if you get busted shouldering that thing, even in the heat of the moment it's a big no no. But this is the interweb, do you own investigating and research on local, state and federal laws to be sure. You might be better off just getting the Tax Stamp for a true SBR.
The price of 300Blk ammo is what kept me from building one in that caliber when I put my AR together. I still might build an upper for it yet just to play around with from time to time.