Way back when, I used 275gr. G/C cast bullets from an RCBS mould in my BRNO, driven to 2,700fps. and achieved 1 1/2" groups at 100 yards. The lube was LBT Blue Lube. These bullets were standard 12 Brinel WW, which were hardened to approx. 30 Brinel. I have also driven 12 Brinel .45cal G/C bullets (not hardened further) to 2,200fps, using 60%BW 40%Vaseline for lube and achieved 1" accuracy at 100 meters and no leading.
I think that with a good modern lube, normal 16 Brinel (Lyman#2alloy) bullets in .366" should be able to handle 2,250fps. 10:1lead/tin is also 16 Brinel, seems to me.