I would 'guess' that for rounds for rifles, modern weapons are generally 1/2 to 1/3 the size of the diameter of a CW round (.223 for M-16, .577 for Enfield, etc). For pistols modern round might be fairly close to a historic round (9 mm for Baretta, .36 for Remington/Colt (heck just had a brain freeze!!)),
JIM T