my background... i live on and hunt the wild, wind-blown prairies of south dakota and wyoming. i do a prairie dog shoot about every 10 days or so, depending on weather. ie, recently we have been in constant rain, so my last 2 dog shoots have been rained out. that's ok... i'll do it this weekend. and the one after that. and probably the one after that, too.
whether or not you want the opinion of a serious dogger, i don't know, but i'll post some of my thoughts...
first, the platform... i don't know what exactly you are getting at here w/ the non-ar semi-auto stuff. the ar-15 is an accurate weapon, and does well on a dog town w/ 20 round and smaller magazines (for when you go prone)... you will only want to shoot maybe 2-3 rounds or so at a time anyway to stave off barrel heat and shooting out your throat on your first trip. but, i also think there are better rifles available - or at least not worse rifles. at any rate, think very seriously about a remington 700 vls. the laminate stock is inletted well and is extremely stable. bed the action ($25 for materials), float the barrel (free), tune the trigger (free) and you're in.
for your factory loads criteria, look very hard at the 22-250 w/ hornady loads, or 223 w/ black hills red-box. shoot 50 or 55 grain v-max's for the 22-250, and 45 grain v-max's or 52 grain matchkings in the 223.
i would also advise against using this rifle for deer. if you absolutely have to go that route, you'll end up w/ a compromise rifle that will neither hunt deer well, nor shoot dogs well. but, if you must, look to the 243, 25-06, and 257 roberts (the roberts is the least desireable because it usually means a long action chambering w/ a short-action cartridge) in a sporter rifle. you will grow tired of the recoil on a dog town, but if you choose a 22-centerfire to mitigate the recoil, you may find yourself not legal for big game depending on your area (i'll leave the ethics argument out, here).
don't skimp on your optics. nothing will turn you off of varminting faster than having a miserable headache because your glass sucks. something like a leupold 4-12 vx-2 or better will get you where you want to be without breaking the bank. if you go the vls route, the 6-18 and the 6.5-20 are aesthetically very pleasing on the big rifle.
as for range, i spent my first 2 years dogging w/ factory loads in a 22-250, and i could consistently and regularly make hits to 600 yards. 400 yards really isn't that far or hard to achieve, even w/ a factory load in a factory gun. i've done it, my hunting partners have done it.
i understand the desire for having something unique or different, but if you go too far out of the box, you will regret it as far as dogging goes.
