A little history, hope you don't mind.

As .615" are the 'bore diameters, ie: tops of the land to land dimension, they are true 20 bore barrels as 20 bore is .615" which equates to a pure lead ball of 350gr. at that diameter. 7000gr. divided by 20 (bore) = 350gr. which happens to be .615" in diameter.

Most European and some English ctg. are named for the bore size ie: 256 Dutch Man. (6.5 groove), .276 Mauser (.284 groove), .303 British (bore size), .350 Rigby (bore size).375 Axite and H&H -(groove diameter). Naming by bore dsize was a hold-over from naming by bore size back in the muzzleloading era. A .45 cal. rifle could have .008" to .025",. yet was still called a .45, not a .466 cal.(.450 with .008" rifling) nor a .50 cal. (.450 + .025" rifling) because a .450" ball was the bore size.

The American method was naming ctgs. by either bore size or groove size - rather sloppily at that, ie: .308 Winchester (groove), .300 Savage(bore), .300 magnum (Bore), .375's(groove), .358's, .450 Watts (bore) and .450 Lott(bore) .458 magnum (groove)

Bore diameter was that size and weight of round ball that would just fit in the bore - a 12 bore rifle (nominally .729/.730" bore @ 583.3gr. ball ) could have up to a .750" groove diameter for .010" rifling.

Back in the patched round ball era with muzzleloaders, a 12 bore shot a 13 bore ball to make room for a 'substancial patch", a 14 bore would shoot a 15 ball, etc, so a 4 would shoot a 5 bore ball.

With breechloaders and rifled barrels shot without cloth patches and not having to have undersized projectiles, they were able to go the other way - all gauges would shoot balls that were groove diameter - so a .010" rifling depth 12 bore would shoot an 11 ball, ie: .750" or with shallower rifling, whatever size fit the grooves, sometimes to the bottom, but often only partically engraved still shot just fine.

Thus, we must slug our barrels to ensure we aren't shooting projeciles too large. A bit too small doesn't hurt the rifling in low volume shooting & can be accurate, however a hard ball (round or elongated) that is larger than groove diameter could if shot in thin barrels.