did she know how to use a gun before?

She's a woman, so of course she is a natural warrior with inborn skill with many weapons.

She's a regular Robin Hood with a bow and arrow, too. She did mention that archery was a big fad among rich ladies in England at the time, and she could hit the bull's eye from 70 yards.

Even accepting that, I don't buy that anyone could pick up someone else's bow with a different kind of arrow and hit someone running away in the brush on the first shot. It would take a while to get used to the bow and the arrows.

And the other Pawnee scout's bow looked more like a toy than the real thing. No way a bow that small could put an arrow right through a person.

Point and shoot, it's not that hard.