For what it's worth, after culling info from several 18th century stores operating in Virginia 1750-1776 (total citation data pool being slim; only 25 guns), it looks like the most common/standard bore sizes for imported English fowlers were 1/2", 3/4" and 5/8" (.50,.75 and .62 or 20g), mostly with 4' or so barrels. Military muskets of course skew to .69-72ish for French and .75-80ish for English & Dutch. English trading guns tended to be roughly 20 gauge.
There is a Rev War era document from the Pennsylvania Committee of Safety that indicates available smooth bores surveyed ranged from 13 (.710) to 30 gauge (.538). Quite a mess to make up cartridges for.