Basically, no. Though many people will cry "the plain guns were all used up!", I think it just isn't so. There ARE plain (even rough) barn guns from PA (and other places, I suppose), primarily in the Lehigh and Berks and Schuylkill county areas. The buttplate-less, lock stock and barrel types (they still exist, and they weren't all used up...). But they only go back MAYBE to, say, 1800. Generally even later than that, though.
My belief is that the reason you find this type of gun in this period is one of economics. With increasing industrialization, and the decreasing price of locks and gun barrels, cheap guns became feasible (also, aesthetics were simply changing, unfortunately...
) . When barrels and locks and all were expensive, and these parts were probably the lion's share of the cost of making a rifle, it made no sense to make something rough and plain when it was going to cost a fortune anyway. Might as well make it a nice one. When rifle barrels began to be produced by barrel mills in much greater quantity, the price naturally would go down, and the cheap rough gun became a viable option.
I got to handle a couple of Berks/Schuylkill county barn guns several years ago. I think they have a charm all their own, but finely finished guns they were not. I made one, myself, though I know I finished it way too nicely!
Not to say that there aren't (possible) pre-Revolutionary rifles that are relatively plain in carving and decoration, they are not rough barn guns, and are complete with the full set of normal brass hardware, buttplates and all.
