You need a special gauge block specific to a five groove barrel. The same applies to any odd number of grooves. There was a machinist several (about 30-35) years ago that made these gauge blocks, but they are un-marked and I do not recall who it was I bought mine from.

Any other method will give you nothing better than a close approximation. Depending on what you are trying to determine. With S&Ws you can get as close an approximation as any method by simply measuring the diameter between the land on one side and groove on the other. For .38-.357 assume a .350 bore, subtract this from what you measured, multiply the difference by 2 and add the .350. So, if your slug measures .354 - .350 = .004 x 2 = .008 + .350 = .358. This should be within +/- .0005".

If you are trying to determine the diameter to size your bullets simply slug the bore, then see if the slug will pass through the cylinder throats with little or no resistance. What you are trying to determine if the cylinder throats are the same size, or smaller that the groove diameter of the barrel. If the slug passes the throats then slug the throats and size your bullets this diameter to that diameter to +.001".

If the bore slug will not pass through the throats then you have to decide if you will live with it, or will enlarge the throats. If you choose to enlarge the throats see above. If you choose to live with the condition, then see above! The best you can do is size throat +.001 either way. Forget sizing to groove diameter, you need to fit the bullets to the throats for best accuracy.

Note, in determining what diameter to size your bullets, nowhere is it necessary to know the groove diameter of the barrel! All that is necessary is to know how the bullets fit the throats.

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Gunsmithing since 1961