I have shot alot of prairie dogs, way over 350yds with a .204 with satisfaction. However, I felt the 204 on coyotes was a bit "light in the britches". I've shot Rock Chucks ( Marmots) out to 250 very readily. But 400yds is a ways out there and for big groundhogs ( I've killed some big marmots) I'm thinking you will wound alot with a fringe hit from the 204. If you want to reach way on out there, I suggest a fast twist 22-250 and the excellent 68gr Hornady OTM on up. A 22-250 Ackley will be a sweety, or a heavy 220 Swift, or even 6mm Remington. Have fun though!
