Algodoo

press up/down to aim.
f to fire.
5m accuracy with 120m of range.

How it works:
bigger/smaller gear transforms barrel angle of a into 2a, then slider extracts y value from point rotated by 2a (that is r * sin 2a). Equation of range is v*v*sin 2a/g. v*v/g is just some constant. (well, v is not entirely constant, this gives some inaccuracy).