Part of the problem is that a lot of these "common" military cartridges came into being in a VERY short time at during the transition from black to smokeless powders and not long after the introduction of breech-loaders.

Whilst early ML rifles required the ball to be "patched" and RAMMED down the barrel to obtain "pre-seating" in the rifling, the Minie bullet was BORE sized BUT had a thin "skirt" at the base end.

This was designed to blow out to GROOVE diameter or thereabouts on ignition. Thus feature did two things: it sealed the circumference for much more efficient use of the propellant gases AND, as the expanded skirt traveled up the barrel, it scraped out some of the residual fouling from the previous round.

These bullets were, nominally, "bore riders".

Look at the numbers for the .303:

Bullet diameter: .311", plus or minus a thou or so.

Bore diameter: .303" to .304", .305" REJECT

GROOVE diameter: out to .320" and still in spec, despite this being a cartridge designed to run at MUCH higher pressures than the old '53 Enfield. The dimensions and tolerances on the BORE are MUCH more "rigorously" defined.

The very hard and thick round-nosed nickel jacket on the early .303 ball ammo indicates it was not meant to do much "blowing out" at launch, nor to be "squeezed" by the lands.

Thus, the .303 was designed as a "bore-rider".

The 6.5 Carcano, being of similar vintage, but not starting out as a BP round, is much the same.