The way to measure a "slug" from a barrel with an uneven number of grooves, is with a special "V anvil" micrometer or with a special "V block"( special for the number of grooves) and common micrometer. You can get a pretty good approximation by measuring with a dial or digital caliper and carefully "rolling" the "slug" until it slips past the jaw of the caliper; or wrap the "slug" with a piece of thin shim stock or a thin blade of a "feeler" gauge, measure it, then subtract twice the thickness of the shim stock.
Mike