.223/5.56 or the ultimate in varmint control the 22-250. Petersons Hunting mag presented this 3,700 fps. round as the best predator cartridge made. 550 ft. lbs of energy at 500 yards where the .223 only comes it at 405 with a 24 inch barrel. A nice bolt action in this caliber would be the end all coyote blaster.
Ammo is around 20 bucks a box for real good ammo so it's right on par for good .223 ammo. It's simply flatter shooting and more accurate than the .223.
Does the "NIGHT VISION" aspect of this question mean anything?
How long is the barrel on that rifle you're thinking of using?
Muzzle flash is a concern when using night vision scopes, right?
I'm thinking that a round based on the .223 case, but necked-up to take a larger bullet in the 6.5mm range, would be best for 100-150 yard shots on coyotes with minimum muzzle flash.
The larger bore diameter (6.5 mm vs. 5.6 mm) means more opportunity for the powder to burn before the bullet pops out of the barrel.
That being said, I think a .223 would be fine if the barrel were normal length (20") OR if a shorter barrel were used in conjunction with a good flash hider. Not a standard M16 bird cage, but a bigger unit. Something maybe 3" long or so, possibly up to a 6" long fake suppressor (they work great for hiding muzzle flash, even if they don't quiet the gun).
I've got a buddy that hunts them and swears buy the 6.8 spc, but I've never used this round. My brother and I used to hunt them all the time and we just used AR's. I think the NV aspect that GAgunLAWbooklet
brought up would depend on your setup. What kind of NV will you be running? Suppressor? That would obviously take care of the flash problem.
Also, the magazine presents a good point. When ammo gets scarce the .223 ammo is going to fly off the shelf and the 22-250 will be a lot more accessible.