I used the French guns for the Dutch-Belgian artillery, from ACTA. Painted grey with black ironwork. Think the Brunswickers will borrow it if not in use!
For Brunswick Horse Artillery, I used British Rocket Artillery bodies, with Brunswick Leib HorseHair plume heads. The pose with a rocket being carried horizontally lends itself to a man carrying a rammer.
Do you mean the set of artillery made by Waterloo 1815? If so, this assumes British-design single trail carriages, which appears not to be what the Brunswickers (or. for that matter, the Dutch or Belgians) had in 1815. There is some helpful commentary here: http://www.plasticsoldierreview.com/Review.aspx?id=326