What was the avg. muzzle velocity of a 1800 flintlock musket?

I doubt there is an accurate answer to be had on this one. There are too many variables. The load of gun powder as well as the quality are substantial factors as well as the size of the shot. I will tell you though, that a flint lock musket was horrible when it comes to accuracy. Anything greater than say, thirty yards stands a good chance of being completely missed, hence, "Don't shoot until you see the whites of their eyes!"