Steve,
Several years back I made a 303 target rifle on a P14 action using a Shilen 311 barrel. It worked great and I believe it may have been the most accurate 303 in the world. I fired five shot groups under 3/8" at 100 and fired possibles on the 300 meter ISSF target. This happy state of affairs lasted a couple of years until Sierra, for whatever reason, started producing 174 grain bullets which were about .0005" larger than they had been (initially they measured 3112" while the next bunch I bought are 3117". Under normal circumstances, this would n't be a big deal but I cut my chamber with a 3115" throat. This was great with the first bullets but the next bunch would push back into the neck rather than entering the throat as the round was chambered. Accuracy suffered to the extent that I was frustrated with the rifle and pulled the barrel and turned it down for use on a Lee Enfield sporter (worked fine).
The action and stock languished in the corner for a few years then, this year, I decided to resurrect it as a 30-03 British. I removed the throat portion of the reamer entirely with the intention of cutting the throat separately and chambered a Smith gain twist and throated it for the 175 Sierras seated to just above the neck/shoulder juncture. This particular barrel took considerable effort break in but now has quit fouling and serious testing can begin. I have high hopes.
I had thought to chamber it for the Epps as a 30-03 Epps Improved but the standard cartridge will produce enough velocity to satisfy me. I have a thirty caliber Epps on a Ruger No 1 and like it a lot but for target use, if I want a 30/06, I have one.
Anyway, the 30-03 British makes so much sense I am embarrassed that it took me so long to do it. The only possible issue is that one can't use factory 303 ammo in it. I have re-barreled a couple of No. 4's to 30/40 Krag and like it fine but the 303 case works better.
One other interesting fact is that 303 brass is thin enough that my 303 Epps die doesn't size the necks sufficiently to hold 30 caliber bullets. My standard 303 dies are fine but the Epps dies (RCBS) are a little bigger. So I ended up making a neck sizing die which uses a Reddding bushing for my 30-03 Epps. Do enjoy your project; it will work well, i'm sure. GD