Longest range shot I know of with a flintlock rifle was about 600 yards. Look for article in Nov. 2003 edition of Muzzle Blasts Magazine. This is a rarity though and you'll learn more details when you read the article.
BTW, I know of several 300 or 400 yard shots. George Hanger talk of one during the Revolution. He was consulting with Lt. Col. Banastre Tarleton (the fellow "Tarlington" of "Patriot" was styled after) when an American rifleman was seen at a distance. The impudent fellow went supine and rested his gun on his foot and fired away. The bugleman's horse was behind that of Tarleton & Hanger. He suddenly dropped off and annouced to the two officers, "My horse is hit, sirs!" Sure enough, the horse then dropped and Hanger then suggested that he and Tarleton should retire before they provide more amusement to the riflemen. Hanger has paced the distance from where he and the rifleman were and swears that it was 400 yards.
During the Peninsular War one 5/60 Royal American was being harassed by a Frenchman at 400 yards. He got tired and asked an officer for permission to fire. Wearily and not thinking it possible, he consented and the German (with exception of the officers, most of 5/60 were actually Germans) fired away and his opponent was seen to fall.
If you want more of this type of stuff (with cites to sources, wait for my book). It's not academic but filled with "fun" little snippets of people blasting each other from afar. Inasmuch as I can, I try to quote directly from the source, complete with original spelling, punctuation and grammatical error, and leave the interpretation to the reader (Wallace Gusler of Williamsburg is doing the same thing with his book on the Virginia long rifle).