IIRC, in Nebraska, the 'minimum' legal gun is .22 caliber centerfire, making a minimum of 1000 ftlbs energy @100yds. The .223 just *squeaks* over that limit. In general, I'd say that if you have the choice, no, a .223, or any .22 caliber shouldn't be the first choice for hunting deer, be it whitetail or muley. That said, I know a *lot* of deer go down every year to .22-250s and .220 Swifts, and probably a fair number to .223s as well. About the only reason I'd let it slide there is people buy their gun mostly for coyote and pest control, and make maybe one shot a year at deer. From that view, why have a rifle sitting around all year collecting dust and rust, for one shot, when you can just take a bit better aim and make one gun do it all. Yes, the argument can be made for just going up in caliber to a .243, .25-06, etc. but stuff like the .223 and .22-250 are cheaper to feed year round, and can be found about anywhere. Not saying its right, just saying that its not wrong either. Just depends on which side of the fence you happen to be on.
Monte