commun formula is V = 1000 d²
but shape of case can make difference short fat case with sharp shoulder are more efficient ex :
6 Norma BR compare to 243 WIN
22 Dasher comapre to 22.250 Rem
basic idea is when you reach max water capacity for one caliber exemple 70 grain of water in 6.5 cal if you go over your gain in velocity became low compareto the increase of powder volume and if you go to extreme you sometime lose velocity or burn powder for nothing
exemple :
7 STW and 7/378 Wea
so except for fun or fashion better to use a longer barrel than a hudge quantity of powder and avoid by this fact carbon fouling effect wich is very commun with real overbore cartridge
good shooting
DAN TEC