The best way to measure your barrel diameter is to slug the barrel. That means driving a dead soft piece of lead (usually a fishing sinker) into the barrel and measuring the widest part of the bullet. Cast and size the bullets to be at least 0.001" wider so you get a good seal.

This is an excellent book to read and it's free. You can also download the entire book in PDF format for offline reading. It's called "From Ingot to Target"

Another wealth of information is at the Castboolit site