I don’t know the answer to this, but some years ago I wandered through the Prattigau when the cantonal shooting festival was taking place. I very rapidly learnt why shooting ranges are marked on Swiss Topo maps. So this question should be of fairly general interest. See http://wiki.openstreetmap.org/wiki/Tag:sport%3Dshooting. The notion of some kind of barrier node at the point of the notices seems sensible. An access tag with a value like “conditional” on the way would be another (non-exclusive) possibility.