I love my 243s for coyote calling! I use a 60gr Sierra HP behind a max load of Varget, and out of a ton of coyotes, I've only wrecked one fur with this load. About 90% of the time you can't even see the entrance wound or any blood on the snow. This is on alot of dogs shot at well under 100 yards.

I've never owned or been around a Cooper, but I thought I read where they came out with a repeater now, so you might want to check into that.

As for AR15s. I can't seem to put mine down this year, shooting 55gr BTs (not going to use them after this year on coyotes again) I'm putting 5 shots into a ragged hole at 100 yards off my shooting sticks. I'm partial to the 55gr SPs or 55gr Vmaxs for coyotes out of the 223. I might try out some 50gr V-Maxs this summer and see how they perform on dogs.