Rotation and attachments

I'm very fuzzy on the way rotations work on objects which are attached to avatars.

What I'm trying to achieve is an attachment which rotates (all of itself, not just one prim), but around a center that is offset some distance from the actual attach point. Think 'mounted on a tripod' (it's a weapon mount for a mech avatar, intended to be able to point anywhere in a hemisphere). I've tried a bunch of wild guesses and nothing seems to behave as I want it to.

Any suggestions for how this might be achieved? I'm probably missing some 'obvious' bit of hilarious rotation math..

IIRC attament roots rotate relative to thei attach point (although it might be relative to their attach offset) and child prims rotate relative to their root objects, with the note that you can't actualy get the roots rotation, because it reports as the avatars rotation (or null, I can't remember now). none of which take into consideration any animations playing, it's all treated as if the av was standing in place, which can serious mess with anything that wants to be relative to inworld objects.