Ideally the neck portion of the chamber should transition to a proper bullet dia. throat, in these modern times usually called a 'ball seat', which then transitions to a leade which takes the bullet into the rifling.
Ideally......
IIRC, many of the old, more traditional cartridges (in. the 30-30) come off the case mouth and pretty abruptly taper into the rifling. Most like this will benefit from a better designed 'throating reamer (check with 4-D!!!) and they will likely have some other questions for you.
FWIW, I have been able to get all my rifles, older, and modern, to shoot cast bullets pretty well by using brass that has been fire-formed to each chamber, not resized, chamfer the case mouth edge a bit, bell a tad, and seat a bullet that is just a 'light' press in fir. If it takes too much press in, it expands the brass neck too much and they usually get sticky. This is how I find the limits of what my existing chamber can handle. It is a little futzy, I admit, but the methodology is sound, and the results have been satisfying. Keep in mind that brass from different brands may have thicker/thinner necks! If you have a couple of brands, you might prep some of each to 'fine tune' your chamber fit with the bullets you have.
After that its usually a plain base up to 1450fps, GC can go faster, made of the same basic WWt + a dash of tin alloy bullets.
I suffered through a Martini Enfield .303 for over a year, that would do 1-2 ft groups at 50yd. at first. When I finished up (almost a novel, not a short story) I had 1-2" with open sights.