Best bullet weight for a 45-70 w/20" twist?

[Fatelvis]

I just bought a Browning 1885 that has 1:20" twist, and was wondering, what would the max bullet weight be for this twist barrel? I know the BPCR version has a 1:18" twist, but was wondering if the little slower twist would limit my choices. Thanks-

 

[Jim Watson]

Trapdoor Springfields had 20" twist and handled 500 gr roundnose ok. The 520-540 gr BPCR bullets might be a little much, though.