How big is the target?
A grid of break beam sensors would be another solution - but the beams would have to be really close together (about 90% of the size of a bb, which is pretty small)/
Or a scanning laser sensor like those used in shooting chronographs:
They scan fast enough to catch the moment the ball impacts, and if you have two or more of them that overlap you can triangulate where the ball hit the target. Two of them opposite one another would result in a roughly square area where they're overlapping.