For max effectiveness, it’s about expansion/fragmentation velocity threshold for the bullet you are firing, the muzzle velocity of that bullet out of your gun, and the distance of the threat. For example, 75 gold dot (a .223 loading) was designed with consideration for SBR’s. 1950 FPS is the expansion threshold for the round per the Speer rep I spoke with. Out of a 10” barrel, you’ll probably maintain 1950+ to at least 100 yards, but not likely to 200 yards. If you want to go with PMC I’d see if they are loading with the Hornady T1 75 grain BTHP and find out its expansion velocity and figure what distance you’ll likely achieve that velocity with your short barrel.
Also, if you want a 5.56 load that performs well out of SBRs (at least at close range) Federal’s 62 grain tac bonded is available online for 1.10 per shot (not sure if that’s within your budget).
Available here: https://www.bonefroggunclub.co...al-bonded-xm556sbct3