Marc - your alloy has to be soft enough to have "some" obturation.

Rod, I havne't seen any green box .303's for quite a while, around here.

The .312" Hornady 174's, out of my (Ken Waters).312 Express shoot very well indeed. 1 1/2" at 100 meters. I have not put them into an animal yet. That round is chambered in my #4 & does 2,974fps with that bullet. Likely a bit fast for it inside 150 meters.
This 2-groove barrel is .315" groove to groove.