Reloading .303 British : You going to do what!?!?

Sounds like an interesting project. In Hatcher's book he writes about the M17 barrels being slightly tighter than the 03 in 30 cal because of Enfield form of rifling and the combined width of grooves are three times as wide as the lands of the latter.

Also by applying a firm crimp on the right bullet style with a slightly smaller diameter bullet, the heel of the bullet could be expanded further at peak pressure and just before moving forward becoming "upset"  as supported by information provided to us in the textbook of small arms.

A lead drawn wire of the appropriate hardness, set into the envelope material to allow the heel to expand in a way that provides a nice gas seal the moment before forcement occurs.........powder selection would probably be of help here too.