The .303 has been 'catted to about everything.....Im sure I remember reading about a .30cal. version with the right size neck and throat for our USA std. bullets. Check with 4-D for the right reamer for this and go for it, but remember, your std .303 size die will likely not work, if LEE has the Collet Neck Size Die for .303 that might be the low cost solution.
I had a large frame Martini Enfield in .303 and remember looking at all the std. .30cal. bullets for sale cheap on gunshow tables, and not so many for the .303 (though I tried some pulled 7.62X39 120ish and they were great for the lighter end of the scale).