People have built 375 Winchester guns on Reminton 788s that started life as 30-30s. I have one in 38-55 (basically same case just loaded to lower pressures).
Lots of guns out there that can be converted to feed rimmed cases - P14 Enfields are easy and can handle high pressure rounds, Siamese Mausers are easy to convert to 45-70, etc.
Not much hope on a Savage (other than the 340 in 30-30 but I wouldn't put a bunch of money in one of those).
None of the rounds really have a "flat trajectory" out to 300 yards. Just get a mil dot scope, and learn your load. With just a bit of practice it is easy to make hits out to 600 yards with my black powder cartridge rifles (40-70, 45-70, 50-70 etc.) and aperture sights - just have to learn to judge the range and know your "come up".
Single shots would certainly be easier (Ruger makes the No. 1 in 460S&W and 480 Ruger now - if they made it in 500 S&W one would be in my safe tomorrow!).
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