i have done something similar in nature, but with much broader applicability. every pawn shop in the universe has buckets of old single socket wrenches, also found in resale/thrift stores/yard sales/etc...and likely several in your own shop. various different brands, 6 point/12 point, thick wall/thin wall will all vary in their outside diameters, even in the same internal size.
take your 0-1" micrometer and measure the outside diameters, and mark that diameter on the socket with an indelible marker and arrange them - mine are in ascending/descending sizes in a board with shallow holes drilled - and each position marked with the corresponding diameter. add to that a few extensions; 2", 6", 8", so forth - i now have 35-40 sockets and extensions that will let me piece together handles up to 10"....and i doubt that i have spent $25 on the entire set-up.
i have 12's, 16's (and one 1884 10 gauge) - so i'm trying to cover spans from a tight full choke to a hone job that's 10 thou over original bore size. i.e., in a 12 built with a .720' bore and 40 thou choke...as tight as .680 - to a 10 thou hone on a .729 bore...as large as .740. my 16 gauge assortment runs from about .620" on the tight end, to about .680". i am trying to fill in the gaps, and to have a plug at least every 3 thou between these points....i can tell you that a difference of 2 thou in diameter is a pretty small difference. for my 10 gauge i only have specific sizes pertaining to a single gun.
so, with a 8" extension and a socket with about a 1" length, you have a decent replica of the english plug gauge, reading 9" from the breech - just like it's done in b'ham. if you have a socket that corresponds to the end of chamber diameter - you have a similar depth gauge for the chamber length that you have established with a coin...but it's much easier to handle securely. and with a set of closely spaced sockets, you have an accurate means of measuring chokes. i have even straightened up some slightly dented (out of round) muzzles - just start with the smallest plug that won't pass into the muzzle...use a bit of grease or wax (i favor frog lube) and gently step up until the dent is lifted.
