I don't load. 303 but do for 7.62x54 which has a similar case capacity. With a 185gr cast and 36 gr of N140 I get 2034 fps which is maximum velocity for accuracy in my rifle. RL7 is very good, start at 26 gr and work up. What bullet design are you using? Most moulds for. 303 and 7.62 Russian are 180 gr and heavier.