I have owned six 45-70 rifles over the years, and now have 2 of those. Despite SAAMI's groove diameter spec of .457", none of those six rifles or any of the several others I have slugged for friends had .457" throats or grooves. Throats are usually at .4585"-.459", and an old Trapdoor I am messing with currently has a .461" throat. Throat diameter is not a measurement that I would "guess" at, or assume about. Ya gotta slug the throat or do a Cerrosafe casting of that area. Few calibers have that sort of dimensional predictability or consistency.
Like a lot of other black-powder-era calibers (especially in leverguns), the throats in 45/70s are usually quite short and feature very abrupt leade angles. This can interfere with chambering when a bullet ogive or its front drive band collide with that leade. A lot of 45 caliber rifle bullet designs take this into account; some others don't. You can have the throat and leade modified, but most folks just trim their cases back a slight amount to achieve better fit with bullet selections when such adjustments are indicated.
Another quirk of BP era calibers and the bullets meant for them is the bullets' lack of crimping grooves placed properly to secure bullets during their shunting trip down a tubular magazine. Unsecured bullets can "Telescope" on you, usually seating themselves more deeply if uncrimped. In BP days, bullets were seated atop a compressed column of black powder, and a roll crimp was set around the ogive radius origin. This wrapped the package nice and tight. Most smokeless powders do not fill the powder space fully in the 45/70, so a crimp groove is a good idea to secure the bullet. At the same time, be mindful of the bullet's interface with the leade as the loaded round is chambered.
Short answer--there can be some gymnastics involved with bullet selection in the black-powder-era calibers. They can be Special Needs Cartridges.
