Just recently been trying some offcuts of fireproof plasterboard (I'm sure ordinary plasterboard would be the same) with both 12ftlb and FAC rifles and the results are good. When hit the board gives off a 'puff' of white dust from the back so a hit is quite visible at range. Because of the backing the board doesn't fall apart but even with the standard air rifle the front face shows a very clean and well defined 'bullet' hole and so a piece of board is useable for some time. Haven't had a chance to shoot the board at more than 40-50m so far but, assuming complete penetration by a standard rifle, it seems to me plasterboard pieces used as a kill zone on a target, or even with a target stuck on, would give a good visual indication of a hit at good distances. Certainly with FAC rifles easily visible hits are possible at good distances. If you have some old bits of plasterboard lying around have a go.