What are the Proof Marks on this rifle, are they Black Powder or Nitro powder. The .303 British round was BP loaded at first it was only later that it changed to Nitro Powder. The Black Powder round had a pre formed BP pellet inserted during the case forming process so loading loose BP into the .303 round will just cause it to spill over the case mouth.

As others have said check the twist first and then work out what bullet can be stabilised in that twist, and go from there.

Harry.

Biology is the only science where multiplication can be achieved by division.