Lovely rifle, Tim. Looks like a find.
Yes, .303s from this period were proved with Rifleite, an early smokeless rifle propellant from the Smokeless Powder Company, because that's what the rules of proof of that time specified for the .303 (the proof load was 46.3 grains Rifleite and a 287 grain lead plug wrapped in greased paper). That doesn't mean that it was regulated with it. Unless otherwise engraved elsewhere on the gun (as opposed to stamped by the proof house), it was almost certainly regulated with standard 215 grain ammunition loaded with Cordite and a nickel jacketed RN bullet.
Keep in mind that the "standard" 215 grain loading of the .303 was increased shortly after the turn of the century - after this rifle was made, if you're right about the date. The original nitro loading of the .303 was a 215 grain bullet at 1,970 fps from a 25" barrel. The new "standard" was increased to produce 2,050 fps in a 25" barrel. Your rifle was probably regulated with the original standard load. You're a double rifle shooter, so you understand the difference that velocity can make in regulation.
Current handloading data for the 215 grain bullet isn't hard to find, but even most listed starting loads are faster than the standard velocity that this gun was regulated at, so you'll need to reduce further. For example, ADI lists a starting load of 36 grains Varget at 2,130 fps, or 42 grains H4350 at 2,045 fps, both from 24" barrels. Start lower, and work up shooting over a chronograph. If the barrels aren't together by the time you reach 2,000 fps, I'd try another powder. I haven't used it, but I would think RL 15 would also work well. Maybe a starting load of 32 grains and max around 36 or 37.
Nice rifle. Best of luck with it.
