Barrel groove diameter is not the parameter with which you should be concerned. Also, it sounds like you may be driving a relatively hard cast bullet through your barrel. When you slug a barrel or a chamber throat you should use pure lead, which is dead soft.
Chamber throat diameter is the measurement you should take. You can drive a pure lead slug through each of the throats and you can use them to determine which is the largest. You need a micrometer, as the precision and accuracy of a caliper are insufficient. However, there are better ways of making the measurement. Some use pin gauges, and just try them until you find the largest one that will enter all throats.
I use a Starrett Small Hole Gage. It measures in only one direction, and so I can determine if a throat is round or oblong.
Attachment 322961
Cast bullets should fit the largest throat.