Hi Pete
buying a rimfire that's the first mistake. As for a tuner well there is alot of information about this here is a link to one method
http://www.rimfirecentral.com/forums/showthread.php?t=544427I did a method in Brisbane which was very good but having done only a little rimfire myself i believe it is all in the AMMO that you get pick the right ammo for your rifle and match as best you can then put a tuner on and see if it makes it better or worse. in the long run if you find ammo that matches your rifle by batch number mortgage your house and get it all it may only ever come once.
cheers Les
shooting well is more a mental control of your thoughts than just pulling the trigger........