If you look at the early flintlocks, they were actually built for sporting purposes rather than military. So although the British Army was using muskets like the Brown Bess from the early 1700's, they didn't trial rifles until the Ferguson in the late 1770's and the first widespread adoption of the rifle didn't come until the Baker rifle from 1800 onwards.
So the early flintlock rifles would still be the place to look.
I am very interested in what firearms the Scots had then, and how common they were, what they cost, etc.
The British Army preferred muskets because they were less expensive to make and faster to reload. Major Ferguson took his own breechloading flintlock to America as a commander of British troops there, and was a very good shot with it. He was killed at the battle of King' Mountain, South Carolina, during an attack by a mix of local militia and Overmountain Men from Tennessee and North Carolina, who had marched all night from the Cowpens, about forty miles away, for a surprise dawn attack. These men were armed with "Pennsylvania long rifles" made by German and Swiss gunsmiths. Ferguson was hit by perhaps as many as twenty shots of .45 and .32 caliber ball from 100 to 150 yards, before he could fall to the ground. But it would not be until the 1840s that the U.S. state militias would start using rifles ( there was no standing army until 1861, no national guard until 1912).
The reason I go into this is that almost all the American combatants that day were Scots who had migrated from Pennsylvania to Virginia and the Carolinas prior to 1760, or recent Ulster Scots come by ship to Charleston for free land after the treaty with the Indians in 1769. Some of the muskets brought from Scotland have barrels or actions from Germany, and some are marked from Dundee.
A local library has a collection of rare books, including hunting books from England. One of them was published during the reign of Elizabeth I, and shows her in a woodblock cut, riding a horse through the woods with a firearm of some sort, and another of her using bow and arrow. A local gunshop here has an over-under British flintlock smoothbore for sale, in .58 caliber, suitable for ball or shot. I will go take a photo of it. A local museum has local firearms from colonists, from the 1620s onward, including matchlocks, then flintlock rifles beginning in the mid-1770s.