Like said before usually lead will be a bit bigger than jacketed because of the fact that lead is softer and will form to the barrel better.
In the interest of finding the best accuracy out of your riffle, yeah slugging the barrel would work great. It would give you the exact dimensions of your barrel with out all the guess work.
For me on the other hand, I just shot the different sizes and figured out what worked the best. My Marlin likes the .459 lead bullets. A lot. It will shoot jacketed rounds fine (.458), but finding good loads isn't nearly as easy as it is for .459 sized lead bullets.
Every barrel is different, just have to find out what it likes.