P.S.
If one looks at Parker Bullets website, one will note bullets for 50 rifles with BC exceeding 0.265. These i didn't mention, because in my testing at 200 yard, it was found the published BC were quite optimistic. Reading on the website tells the BC were calculated at 2800 fps. The fps from a 50 rifle using 3 pellets pyrodex will be quite some less than 2800 fps. Testing from my 50, at 200 yard, showed the BC of these bullets was considerably less than published, and i do mean considerably. No testing was done with the 50, at 300 yard, because the 45 works so very much better out there.
Another source for high BC bullets may be CE bullet, but their bullets are all copper, and they are therefore kinda long, which require a faster than 1:28 twist.
One can read about higher BC custom bullets on the smokeless websites, however the speeds those boys achieve is much higher than a smoker will produce. The BC of their bullets at those speeds, are a way higher than they would be from a smoker.