Wow, what a beautiful piece - love the lines of it. Something about the style seems later to me than 1777, but it's not impossible. The wrist and buttstock looks like Eastern Berks, Lehigh-ish, but closer to 1790s. The ramrod pipe is reminiscent of Bucks co guns. While the rifle has a bunch of neat features from different areas, the lock has me puzzled. The shape is odd, and the grooves filed into the tail look secondary - especially to a round-faced lock. What do the insides look like? My hunch is a reconversion or maybe a replacement lock sometime. Sorry I can't tell much more - looks like a great find!! Congrats to you!
-Eric