When slugging your bore you may get an undersized reading from the odd number of "spires and flats" (for henry patent rifling, rather than lands and grooves for the kind of rifling we're used to now). You'd be measuring spire to flat; try rotating the slug between the caliper jaws to see what its maximum diameter is, or better stil, try ring guages if they're available.
It's a trap a lot of Martini Henry enthusiasts run into when slugging their bores.
RG