One can, with some difficulty, slug the bore just ahead of the chamber by blocking off the end of the rifling using, say, a solid cylinder of some kind (hardwood dowel, metal rod, or the like) of sufficient diameter to enter the chamber all the way but otherwise so close-fitting as to be stopped against the rifling, and of the exact length that it is held firmly in place against the rifling by the closed breechblock. One would then push a close-fitting lead slug into the rifling from the chamber with this implement, close the breechblock, and then use a suitable rod hammered from the muzzle end to swell the plug fully into the rifling just ahead of the chamber ... then open the block and drive it out into the chamber ...
Essentially the same problem is encountered in trying to slug the bore of a Martini-Henry, which has a tapered "leade" into the main part of the bore ... as can be seen from this diagram:
I realize this diagram is a bit hard to interpret ... however, as I understand it, the "standard spec" measurements are:
Rifling diameter (i.e. measured at deepest part of the rifling)
a) just ahead of the chamber: .469"
b) 11" from the breech: .464"
(i.e. a diameter reduction of .005")
Bore diameter (i.e. measured at top of lands)
a) 4" from the breech: .451"
b) 8" from the breech: .45"
c) 11" from the breech: .449"
(i.e. a diameter reduction of .002"
Another important factor to keep in mind when slugging the bore of either
a Snider-Enfield or Martini-Henry is that they both have an odd number of lands and grooves (either 3 or 5 for the Snider, depending on model, and 7 for the Martini.) Accordingly, snugging a calliper down onto the slug will not
give an accurate measurement of effective bore diameter, because the "low point" on the slug left by a rifling land will always be directly opposite the "high point" left by a rifling groove ... whereas what you need to try to measure is the "major diameter" - i.e. a circle intersecting the bottoms of the rifling grooves all the way around. If you are using a normal calliper, you can approximate this diameter by having the jaws of the calliper relaxed somewhat so that "rolling" the slug between them just brushes them all the way around ...
What I am trying to explain is fairly easy to see with this somewhat exaggerated diagram of the simple 3-groove Enfield rifling of a three-band rifle ... the measurement on the left is what you get if you just clamp the slug in the jaws of the caliber, while it is the measurement on the right you must try to get -
The same consideration is at play when trying to slug a Martini-Henry bore, although its Henry-pattern rifling is quite a complex design. Rather than consisting of simple lands and grooves it is essentially a modified form of Whitworth rifling: consisting of a "septagonal" bore (i.e seven-sided) with a little ridge right down the middle of each of the seven main valleys, the peaks of those seven ridges intersecting exactly the same circumference as the mid-point of each of the seven "flats". Note that this diagram gives the same "bore" and "groove" diameters as the above cross-sectional diagram gives at 11' from the breech -
Actually, the above diagram is an oversimplification of Henry rifling ... but let's not go there ...