We need to clarify something , it's the " groove " diameter , not the bore diameter of the barrel that's important . The bore diameter is the smaller of the 2 measurements . Measuring even the newer smiths is not a big problem . Yes , the 5 groove rifling does " slightly " effect the measurement reading but it is still important enough to obtain .
The first measurement is the barrel as that is the last thing that the bullet must fit to . The accuracy is from the barrel , not the cylinder .
After verifying the groove diameter and add at least .0001 thousandths to that meaurement this is the size cast bullet you want . THEN--- fit the cylinder throats to that size bullet , not the other way around . It should push through the cylinder with firm finger pressure , not using a hammer and a stick . In my 25-5 , I shoot .454 cast bullets with great accuracy . The cylinder throats were already oversize which did not create a problem . Oversize is better than undersize as it won't " down size " your cast bullet .
Veral Smith @ LBT Molds sells a soft back small booklet for about $15 explaining all this in great detail called , " Jacketed Performance with Cast Bullets " . You can email him as well . I have bought his book and corresponded with him numerous times over the years and learned much . His molds are top notch and he is a wealth of information on shooting cast bullets in rifles , revolvers and semi auto's . I hope this helps clarify some of your questions . Good Luck , Cowboy4evr