A true 4 bore round ball weighs 1/4 pound, which is 1,750gr. (7,000gr. divided by 4) & is 1.052" in diameter.

If 1,500gr. is correct for it's projectile, why? The reduction in weight must come at the expense of a hollow projectile which would penetrate less in comparrison to a solid round ball as the lighter, hollow bullet has lower sectional density, or if the gun has a much smaller, non-4 bore, bore size?

A 1,500gr. round ball is well under 1" in diameter - about .985", where as 4 bore is 1.052". What is the actual diameter of a 'solid' 1,500gr. slug - would it be closer to 7 or maybe 6 bore? Either would make more sense.

See mauser- just saved your retinas with a little 6 bore.