It is strange that Ruger chose to produce barrels which were so large. Perhaps they did this to accomodate Hornady bullets which are .312. Winchester bullets I measured are .310. Some old CIL 215 grain are 3105 while some Norma 216's are 311 even. Sierras are 311.
I had toyed with the idea of building a Number One in 303 for myself but decided to build it in 303 Epps and use a 30 caliber barrel. I reduced the throat on the Epps reamer to .3085 dia. Going with the Epps assures that I won't accidentally put a hot load into one of the Lee Enfields.
In the Number One Ruger, the 303 can easily be loaded to match the 308 Winchester but, if you have a Lee Enfiled in the house, you want to be certain you won't mix the ammunition. One way to do this would to be sure loads for the Ruger are too long to fit into the Lee Enfield magazine. GD