The Ishapore .308 rifles were designed to handle the 48,000 CUP NATO standard Ball loads. The .303 equivalent, in pressure, would be the Mk 8Z loads.
I commonly use a .303 load with a 150 grain Hornady j word at 25-2,600 FPS. Using IMR 4320 powder. In my SMLE. Chamber pressure is supposed to be 46,000 CUP.
A similar .308 load should work well in converted mausers and the Ishapore rifles.