With that muzzle brake, I assume this was not intended as some old west, Sharps or Winchester type gun. It is a shame, I would cut that brake off. This looks like it was a somewhat modern take on a British big game or safari class gun. The 50/140 was a good fit case for the action and 510 suitable for bullets used in the 500 3" Nitro Express.
I don know enough about applying loads from A to B. Take this with a grain of salt. I think you could safely substitute 500 NE loads. Those are double rifles loads in your very strong falling block. Your case is 1/4" longer, maybe a little skinnier. Hornady may list some loads. Did you look? I mean for 500 NE loads.
I guess you know Bill Ruger was inspired by a British single for his No1. Sorry I lost to many brain cells to recall. Then I cannot spell the F word.
I suppose without the brake and shooting 500NE loads a full 1" solid rubber pad would be a good addition. More rubber, less brake.
