The term sharpshooter has been around a long time. I did a quick search and it has been in use in the military context of a marksman since at least 1802, long before the advent of the Sharpe's Rifle. One source contends it comes from a German word "scharfschutzen" which sounds like, and I assume translates as, sharpshooter. James Fennimore Cooper used itnin a number of his novels including The Last of the Mohicans and The Pioneers, both written before 1835. I am thinking that I have seen it applied to Rogers' Rangers in the French and Indian War, but maybe that's the wish being father to the thought. I suspect it was widely used on the American frontier in the early years of the 19th and possibly the 18th centuries, and meant precisely what it says.