I want an AR pistol so that I can add an SB15 brace and make it an UN-SBR. I've seen Mr. Consiglio speek of 300 blackout being better for short barrels due to faster powder burn rate, so I thought of building a 300 upper but then I check ammo prices and sent that idea on the nope train headed to screw-that-ville.
So what is the minimum barrel length upper that still maintains some fragmentation range? Are there any factory loads that lend themselves particularly well for SBR's i.e. faster burning powders?