If you are not sure on caliber yet, make your first one a 223. Make sure you have a wylde chamber and a barrel with an 1 in 8 twist. barrel length is largely personal preference. Mine has a 20" wilson barrel. If nothing else 223 is DIRT CHEAP TO SHOOT. with the right bullet choice you can target shoot at the 600 yard line and slay coyotes and most any vermin beyond ranges most folks have any business trying. Once you get your feet wet, with help of some alternative calibers, you can hunt anything with horns and hair if you exorcize common sense and KNOW WHEN NOT TO SQUEEZE THE TRIGGER.
I will forewarn you now, with AR's YOU NEVER STOP AT JUST ONE. It really is a disease.
.308 Howa 1500, varmint supreme. 24" .820 barrel, Roedale precision brake.
.223 Howa 1500, varmint supreme, 24" .820 barrel.
RRA 20" predator pursuite, Magpul PRS, ALG trigger,