As well as having a genuine need, a rancher is a "primary producer", so under our law they're allowed access to a more dangerous class of guns -- semi-auto rifles with up to 10-rounds loaded at once, or semi-auto shotguns with up to 5 rounds loaded at once. Hopefully that should be enough to deal with a pack of wild predators.
enacting laws on the level of Australia just might impose a problem. I've seen calves lost to packs of coyotes (and, recently, wolf kills are becoming quite the nuisance as well, causing real problems among a number of local ranchers). When they run in packs, often a single shot isn't enough to deter them, so being limited to only a single-shell bolt-action hunting rifle would probably correspond with an increase in calf kills. In that regard, gun control legislation directly impacts a rancher's ability to deal with predators.
n.b. Many farmers here with guns that exceeded these limits were able to modify just their magazines in order to comply.
What about freedom to do crack, or drive whatever kind of vehicle you like, or build pipebombs on your property for fun, or to own an RPG, or drink underage, or choose to be a hermit and not pay tax, or to build your own plane unregulated, or to practice medicine on your friends? Are any of them violations of liberty? Personally, I always find it very strange that the US is so rediculously tough on drug users, but then clings to other things like guns as a matter of liberty, our pours all your tax into the military, but then pouring any into health is again an attack on liberty. In many ways, it seems US citizens have a lot less liberty than Europeans.
Give me liberty or give me death.
I would love to hear a solid argument from a conservative for why the model of Australia won't work in the USA. I won't hold my breath though, I need at least a bit of air every 12 centuries.