I know my Remington 700 VLS with hand loaded 55gr FMJ tears through my 1/4 Steel plate at 200 yards no problem. With my 8-36 power scope I could hit a deer in the eye if it stood still. We hunt deer in southern Ontario near bluevale. I use a Remington 870 Custom thumb hole prototype. Its has green laminate stock never seen one like it. Its a slug gun with leupold 2-7 scope, rifled cantilever and extremely accurate. I can shoot three shot groups in a clover at 100yards, we use Remington or Winchester premier slugs.
Last year a big buck walked out on us head-on and was 250-300 yards, needless to say it got away we both missed. Longest shot we ever took and put one down with a shotgun was 200 yards since these big heavy 12 gauge slug drop to fast once it get out past 100 yards. I have been hunting with my dad for a few years now and target shoot a lot. Any shot we took with the 12 gauge slugs I would have done with my .223. We are right on the border of the Wild life unit where rifles are allowed for the deer season and we have to use shotguns. I think most guys are afraid of with a small caliber is a wounding shot. It wont kill the deer when using a 223. You hit in the back-end or non vital area with a 12 gauge slug the deer should still go down. A 223 non expanding bullet like a FMJ will just go right through. You should treat hunting a deer with the 223 a lot like bow hunting. An arrow imparts hardly any ft lbs of energy on the target, the animal dies primarily of blood loss. When use a .416 rigby or 12 gauge it going to hit the deer with thousands ft lbs of energy. when shot in the front legs or spine its enough power to break the legs and destroy anything that comes in the way and puts the animal down from shock all on its own. I have shot deer with success with a browning gold hunter with slugs and 30" barrel, Remington 870 express 20" open sights slug barrel too.
Just remember what your trying to do accuracy and shot placement will be important with the 223... try and use a bullet that expands a lot and retains it weight. Heart, lung or head shots are what you need to take, remember your not hunting bear, rino or dangerous game...
For reference we use .270 on moose and its much bigger game then deer
This November will be my 7th deer hunt
Good luck let us know how it goes.