1941 Winchester .303

[Bruce_in_Oz]

The problem with '303 case-head separations may not all be down to rim thickness.

.303 headspaces on the RIM, not the case shoulder , like sensible rimless cartridges..

Your "surp-mil .303 rifle has had its chamber "refreshed, (to clean up roughness , the brass at the front end will expand more than usual. Depending of the "annealing", if you full-length size your brass for optimal feeding, that could be a LOT of metal fatigue going on there... The usual manifestations, in order of "annoyance" being: cracked necks, blown shoulders and case separations.

Forty to fifty thousand PSI exerts a LOT of encouragement for the brass to "flow".

Likewise, squeezing it all back to SAAMI dimensions in your trusty Rockchucker, makes the brass flow inwards AND forward. (Hence, why experienced / well-trained reloaders TRIM their rifle brass, especially)

IF you have ONE.303 rifle, use the neck-sizer die and the full-length die, . And an annealer. The FLS die is used to do a "general" resizing. It is backed off a few turns. Then the neck-sizer is adjusted slowly until the empty, sized brass JUST drops smoothly into the chamber.

Trim and inside chamfer as desired / needed.

If you have a bunch of different models and conditions of rifle, you have two basic options:

"Custom" case processing for each rifle, or, "universal" sizing (one size fits all), and buy bulk brass when it turns up on "special" sale.

And DO NOT use boat-tailed bullets in ANY .303 Lee-Enfield, Ross or P-14.

Or, buy an Ishapore 2A1 and enjoy shooting without fretting about all that brass management,