I've done a couple of bullet tests on the 223 SB 55 grain soft point. Two does not make much of a sample. Here's the test and what was used.
Milk jugs filled to the top with water backed by packed wet newsprint and the same newsprint alone. Range 100 yards. AR 15 with 20" barrel. Results were: Water jugs(2) penetrated but not damaged to the extent I expected from past tests with 223 55 grain SP followed by 8-9"of newsprint leaving a 1" cavity, bullet nose mushroomed somewhat no evidence of tumbling. W/O water jug, penetration of wet newsprint was 14-15" exiting back of bundle and bullet lost. Maximum cavity appeared to be 1 1/2" about 5-6" into bundle tapering to 1/2" at exit. I think another 2" of paper would have stopped the bullet.
My perception based on this meager testing is that this ammo is as good as could be expected from a 223 regarding penetration but doesn't give me much confidence at to the actual wound produced. These cavities are smaller and penetration greater than what I've seen with US M193 fmj at 100 yards because that bullet generally tumbles and/or breaks apart about 6-8 inches into soft media.
I don't feel the 223 is an adequate deer cartridge regardless of the fact that a lot of deer have fallen to it. There are just too many variables involved to say it will kill every deer hit in a vital spot IN A WAY THAT ALLOWS THE ANIMAL TO BE RECOVERED. That's the catch phrase. A 22lr in the lungs will probably kill a deer at some point but maybe not when or where it can be recovered.
Point of fact: I once found a 1500# cow bleeding from the nostrils and obviously having some sort of problem. A late night visit to the vet did not solve the problem although administration of medication eased the symptoms. Three days later(and additional medications) the cow died in my corral despite my best efforts. Postmortem indicated the cow had been shot in the left lung with a 22lr but the bullet had probably been coughed up in a blood clot at some point. The landlord on this farm had a garden and the bad habit of shooting at starlings sitting on the fence to "protect" his plants. Since we didn't have a bullet, we couldn't prove one of his shots went over the crest of the hill and hit my cow. I'm a pretty tough guy when it comes to animal care but it surely hurt me to see that cow die that way. We kept thinking she was going to make it but lung congestion and blood clots finished her off.
If I'm going to shoot a deer, I intend for it to die quickly in a place I can find and recover it.