Hi Ross,
Welcome to the forum mate. Anthony has hit the nail on the head with 6mmBR and associated wildcats. For what reason they are the most common is simple, they work and are easy to shoot.....by this I mean light recoil and forgiving at the bench.
I would recommend coming to a shoot and have a try of some of our rifles, most guys are more than happy to offer a few shots as we all usually have a fe rounds left over. I think it's the best way to get a feel for the sport.
Regards,
Deane
Just doing my part to reduce the fly population!!!!