Multiple Variable Consideration
DMK +1:
There are multiple variables. Besides the projectile weight, and the velocity, one must also consider the powder quantity, and the powder's burn rate. Also, the old 303 may have a metal butt plate. The 303 a Pachmyer. There is the weight, then too, the stock...wood V. synthetic. Synthetic does give a bit. The stock design also contributes or hinders. Look at Weatherby's old research on cast-off. Too many variables though to say just weight / velocity. I read an interesting article many, many years back that talked about how significant powder burn rate is...punch versus push. Power, now that is a different matters...the 308 clearly wins.
Doc2005