G'day Rimfire gurus,

Can someone please give me very quick run down
on who's rules we in Oz follow regarding Rimfire Benchrest?
As a lapsed Centrefire Benchrest shooter I know SSAA used
to have their own comps., Nationals etc. but it seems those comps
are now being run by one of the Orgs. listed in my posting title.
Now I don't really care who runs the show but can anybody tell me
the dominant Org and the most popular classes?
I have a decent custom Rimfire rifle that, with a few mods, should
fit into one of the classes somewhere but I don't want to go making
permanent changes only to make it legal in a class that very few shoot.

cheers,
Chris