MEASURE BARREL DIAMETER

A rifled barrel has two dimensions in its internal diameter.  There is a land (bore) diameter and a groove diameter.  In threading terms, the bore would be the minor diameter and the diameter at groove depth would be the major diameter. Measuring these diameters can be done easily if the rifle has an equal number of lands and grooves.  If it is a 3 or 5 groove barrel, some calculation is involved, but it is still possible. The method below determines only the minor diameter.

Tap a bullet very lightly a small way into the muzzle of the barrel.

The bullet should just pick up the rifling all the way around its circumference.  Tap it out of the muzzle with a cleaning rod through the bore of the barrel.

This is an eight groove barrel and the bore is imprinted nicely on the bullet.

Slide a vernier onto the ogive of the bullet until the vernier stops against the vertical notches formed on the bullet by the rifling.

The imprint on the bullet measures 0.403" and that is the diameter of the lands of the barrel.  That still leaves the grooves of the barrel as an unknown dimension but, if one dimension is known, we can look up the groove dimension.  A barrel wears more on the lands than in the grooves and, the chances are that the land diameter will show more wear than the groove diameter.