ALWAYS SLUG YOUR BARRELS. If you have any doubts before you do it, you won't after you mic the slugs. Calipers are handy tools, but they are not accurate enough for this job.
I have worked in a factory making match barrels. Even in the best of them there are tolerances of a few thousandths of an inch, and this difference is critical for cast bullet performance and accuracy. Anyone who cannot "see" this difference can't read a micrometer. Geargnasher has it right. (An obviously he can read a mic, too.)
If you want to really learn about shooting cast in a Marlin or any other rifle or pistol, go next door to Cast Boolits. There are some folks on the Marlin site that have a good grasp of the variables, and it's a great place to learn about care, repair, and maintainence of Marlin rifles. However, the EXPERTS on cast bullets hang out next door - the folks from all over the world who design and test new and old bullet designs, and the folks who use them for target shooting out to 1000yds and for the hunting of everything from mice to brown bears. Go where the real knowledge is.