So my original thoughts are along the lines of the .222 for the fact i have brass and dies.... even then i could acquire brass and dies for most little varminter cals so i put the feelers out..
im 'into' the rifles and cartridges these days , in my own place now and can reload whenever, or enough to last the year or whatever.. so its kind of opened the door for something else with in mind my 11 year old son will start shooting the '222' in maybe 12 months or so (loves .22lr) an fundamentals are key so all good there..
Not super keen to end up with having a Cartridge that is too difficult to find brass for, or form easily in say 10 years time though..
I was leaning toward something like the .204 Ruger (same bolt face i believe unmodified) barrel screwed on, in stainless, thoughts here was to flatten the trajectory somewhat noticably out to 300m maybe even say 400m fast and flat. thus will come noise and 'recoil' but i am the one shooting it 98% of the time anyway.
.223 was a little vanilla, but ive learnt with a 308/30-06 that Vanilla tastes great at the end of the day.... thinking it gives me an extra 100m by just holding on.
.17 Rem isnt out of the question, old school, little projectile very fast.. brasss may be a slight issue these days down under... i have simplex Dies but would source some suitable setups as know few older guys with 17s from fox shooting days of old.
finally, the .20 Practical (.20-223) seems legit...
wont modify the bolt at all , an the logical thing would be to put a 222 barrel on it eh!
204 Ruger is basically a 222 Rem Mag necked down to 20 cal, slight changes in the case. 17 Rem is just a 223 necked down to 17. I shoot all four cartridges, and the 222. Around here, if you are looking at easy components to find, the 223 gets the nod. Not my favorite varmint cartridge, but extremely easy to feed. A fast twist and/or the AI version adds a little more to that. Fwiw. - dan