If you ream a rifle throat, don't use a reamer more than .002 larger than nonimal groove, and that with a gentle taper at the end. For jacketed use, the ball seat, or straight part of the throat, should be close to jacketed diameter, even if it doesn't clean up the lands. i.e. Should a 30 cal have a .310 groove diameter, a .3085 ball seat will provide the best accuracy, but the gun won't work well with cast, which has to have a plug fit. The ball seat will have to be a little over groove diameter for cast, and live with decreased accuracy when you want to use jacketed. A good general presumption is that if a rifle has a very large and long throat, the heaviest bullets for the caliber will probably shoot most accurately, because they tip less. This only if the rifling is fast enough for the heaviest bullets in that caliber of coarse.