Agree that your cast bullet should be a "push-fit" or a light tap to push it through the cylinder throats. Usually bullet diameter is about 0.0005"-0.001" larger than groove diameter. The slip-fit through the throats, or a light tapping to get it through, is the best test as the actual measuring of throats is difficult to do properly with i.d. calipers, and is perhaps best performed by commercial plug gauges.
You can drive soft lead fishing sinkers through the throat and then measure with standard mikes. Use the sinkers that have a longitudinal hole all the way through them, that takes the fishing line. Buy the sinker size that is closest in diam to the proposed size of your bullet, but a slight amount larger. If too small, can squeeze it gently in a vise before tapping it through the throat. check out the technical manual by J. Marshall Stanton from Beartooth Bullets Co. Excellent manual for you.