[Unclenick]

Getting some pure lead to slug the bore and then measuring the slugs with a micrometer (not a caliper) that has ten thousandths resolution is a good start. It is pretty common for cast bullets to shoot well at 0.0010" to 0.0020" over groove diameter, but for accuracy to deteriorate as they get bigger than that.

One thing you could try is getting one of Lee's inexpensive 0.309" sizing die kits that uses your reloading press, and running the bullets through to size them down. This may or may not work. It depends on bullets to be good quality and of even hardness and without voids anywhere, otherwise the sizing down can throw them off-axis.

The other issue, if this is not a gun you've used with cast bullets before, is it is not uncommon for some barrels to have constrictions in them. Jacketed bullets don't usually mind much, but cast bullet accuracy is destroyed by them. You often find them in lever guns under sight dovetail cuts or where the barrel screws into the frame. You need to push the pure lead slugs all the way down a lightly oiled bore to feel whether you have any or not? If you do, firelapping the gun is the most common cure, though conventional hand lapping will do it, too.