bigskyhound - the Husky model 20 in 16 bore I had, with one rifled barrel & one smooth, had a .705" groove (13 bore) and .675" bore(15 bore). Both barrels had 16 bore chambers. The right barrel was the rifled barrel, with straight grooves.
It's rifling was .015" deep, per side.

According to Husqvarna, the left barrel was meant for paper case with shot- the right for a brass case and round ball, thus one could easily tell what load one had in their hand without looking at it.

The largest balls I shot i this gun, were .684", which engraved about .010" per side. With the wads beneath, they show fairly well.

What also worked well was a 16 bore ball and a .030" denim patch.

With that in mind, a .620" ball, with .0225" per side denim (10 ounce) would make .667", a nice tight fit in the grooves and nice tight fit (probably) in a brass case.
The nice thing about using cloth patched balls, is the lube (I used Neetsfoot oil) keeps the fouling soft, are very accurate, solves the deep-groove-ball-diameter dilema and there is no buildup of fouling.

Jeff Tanner in the UK would be delighted to bore you a mould, any size you want, and at a reasonable price.

All three types- shot, 16 bore ball and 14 bore ball.

Note- as the target below shows, the 16 bore, patched balls shot better than the bare 14 bore balls. It's definitely worth a try.