Wrap a thin shim stock around the slug ( Not too tight ) A Feeler gauge works well.Measure the shim stock and slug.Subtract double the thickness of the shim and you will have the measurement of the slug.
Thank you, sir. That might just be workable. I'll try it.//// I tried it. Used a 1.5mil feeler gauge, and then also a 0.002" to even out the valleys. Seems like I have a bore diameter than is 0.428". Which, for me, is a good thing, since my cylinder throats are .429". You had a good suggestion.
Sonny