I've got a suppressed 18" Aero Precision AR15 in .223 Wylde that kills stuff pretty dead out to 350 yards. Thats about as comfortable as I feel shooting with it accurately. It's about a 1 MOA gun so at 350 yards that +/- 3.5 inches starts to mean a lot on fox, coyote, and groundhog size critters. I rarely need to shoot further than that, so it has remained a faithful varmint companion for several years now.
<300 yards- .223 caliber AR15
<400 yards- .22-250 bolt action rifle
>400 yards- 6mm Creedmoor bolt action rifle