American rebels in 1775! Well, a variety of arms would've been on hand: their own militia weapons for one, especially "fowler-muskets" which did double duty as hunting guns and military guns (militia statutes required members to own long arms of "musket bore" (.69 cal or larger). Slender, small-bored hunting guns didn't cut it! Composite guns were very common: older military locks, barrels, etc. re-stocked into "new" muskets, often with a mix of English, French, and Dutch parts.
Finally, though they would have had their hands on few issue-type "Brown Bess" muskets (these being in Royal arsenals for use by Crown forces), there would have been large numbers of "commercial grade" copies of the Bess in circulation in the colonies - basically cheaper/simplified versions of the issue Bess....