So I slugged the bore and determined ?????
I am always taken me to task, when casters say "slug" the bore to determine the correct size cast bullet to use.
I say, all that determines is the size of the tightest spot in the barrel, better to start a bullet into the muzzle and make sure the rifling at that point (the last point of contact before exiting) contacts both the top and bottom of the lands. Then size your projectile to that dimension.
Take the humble 303 British;
According to the Small Arms Identification Series by Ian Skennerton "Magazine Lee-Enfield Rifle" Page 29
1) Lee Metford and Lee-Enfield Mk1:
Barrel Specification:
Barrel Length - 30.5"
Rifling Metford - 7 groove L.H. 1 turn on 10"
Rifling Enfield - 5 groove L.H. 1 turn in 10"
Bore Dia - 0.303"
Metford Rifling Depth - 0.004"
Enfield Rifling Depth - 0.0055
Metford Land Width - 0.023"
Enfield Land Width - 0.0936
From book "The British Service Lee" by Ian Skennerton Page 86
2) Short Magazine Lee Enfield Mk 3:
Barrel Specification:
Barrel Length 25.2"
Rifling Enfield - 5 groove L.H. 1 turn in 10"
Bore Dia - 0.303"
Enfield Rifling Depth at muzzle - 0.0065
Enfield Rifling Depth at breech and to within 14" of the muzzle - .005
Enfield Land Width - 0.0936
From book "The British Service Lee" by Ian Skennerton Page 158
3) Short Magazine Lee Enfield Mk 4 & 5:
Barrel Specification:
Barrel Length 25.2"
Rifling Enfield - 5 groove L.H. 1 turn in 10"
Bore Dia - 0.303"
Enfield Rifling Depth at breech - .005
Enfield Land Width - 0.0936
It would seem from this data that an Enfield 303 could be .303" over lands and a bore of up to 0.314" at the breech and up to 0.316 at the muzzle.
My Martini Enfield 303's shoot best with a .314 dia projectile which would seem to fit with the above specifications and bore diameter of .313"