I was playing a game against a friend who was playing Dark Elves. He cast some sort of spell that can reduce the ballistics score of a target. He choose my Organ gun, moving it's BS from a 3 to a 1. However, when I decide to shoot with it, I have an engineer nearby that I use Artillery Master with which says I can use his ballistics skill instead.
So to me it read that my OG's BS went from 3 to 1, but since my Engineer's BS was still at 4 and I use it instead of my OG's BS, that I'd be getting the full 4. I think this is right because the spell says it reduces the BS of the target, not that it gives -2 to hit.
Is my thinking correct? If so, an Engineer really does make it harder at least in that case to reduce the effectiveness of my OG by much (He could have targeted the engineer instead, in which case I would just use the default value).