I think the .243 is the way to go, it is chambered in most rifles now and has a wide selection of factory loads for varmints and antelope/deer. As far as the rifle, if you buy a quality rifle you do not necessarily need a heavy barreled varmint gun to reach out to 400 yards. The heavy barrel really comes into play for prairie dog hunting where you are shooting a lot (hopefully) and it will help dissipate heat and such. I coyote hunt with my .308 deer rifles generally. I just load them up with 150 grain ballistic tips.