Postmanbut

Enjoy this moment, it's almost the best with "the last moment" tinkering, when fastidiously checking everything and doubting. That's especiually true with reloading options, but in my case it was worse AND HIGLHLY ENJOYABLE with customising my bow and arrows.

I don't have a 500 but a 470. At 50 yards the right barrel shoots a little under 2 inches right, with a standard reloading using Woodleigh either soft or solid..........and preferably Federal 215 primers. Sacrificing group for standard oomph.
I recommend that you try other bullets if you can but all in all the best of buff hunting is the approach, and at 50 yards or less, accuracy isn't the main concern. Should a charge happen, you'll be graced as it's very rare and thren it's mainly the PH's job. I have spent more than 35 days just trying to approach ever scampering buffalos.
Keep on brainstorming, it's fun and that makes me feel sure that at the right moment, you'll be fully ready.