Was the gun purchased new? If not are you certain that no one has opened the chokes on the gun.
As far as needing a bore gauge, for years I've successfully used an inside caliper and a 1" micrometer to measure bores and chokes. Typically when I've compared my measurements with a bore gauge they've been within 0.002" or less. The inside caliper is also excellent for measuring chamber lengths. You open it up in the chamber, slide it in until it's stopped by the forcing cone and then mark the length on the caliper with a Sharpie pen, pull it out and measure the distance with a ruler, very easy!
Steve
