An inexperienced deer hunter is probably more accurate with a .223 AR than a 12 ga. 1100. But, IMHO, a .223 is a bottom-end marginal for whitetail deer (at least those deer the size of Iowa whitetails). A 12 ga. slug from a smooth bore shotgun does not have to be as accurately placed as a .223 to take the deer. If a center fire rifle is an option, I would recommend a .243 Win. as the minimum. With a 12 ga. shotgun, it is hard (if not impossible) to beat a fully rifled barrel with a cantilever scope and a sabot slug ( such as those made be Remington. I've taken Iowa whitetails at well over 100 yards (standing, walking & running) with a 12 ga. 870 SP with a rifled barrel, Leupold 1 X 4 scope and 2 3/4" Remington Sabot Slugs.
Edit: Center-fire rifles are extremely limited for deer hunting in Iowa and a heart/lung/liver shot with a slug from a 2 3/4" 12 ga. is not over-kill on a mature Iowa whitetail (at least in the opinion of Iowa legislators and the DNR). In my experience (over 40 years of hunting in Iowa, Missouri, Minnesota, Colorado, Wyoming and Montana for pronghorn, whitetail, mule deer, elk, black bear and varmints) a wound from a 12 ga. slug is similar to a wound from a .243, .270, 7mm Rem Mag., 30-06, or .300 Win Mag. (depending, of course, on the range; e.g., 25 yards or 300 yards.)