You need to talk to a subject area expert.
Suggest you contact the NRA (National Rifle Association) to ask them for a referral to someone who would know who you should talk to.
btw,.223 ammo is considered less powerful than the military 5.56mm ammo. Some manufacturers of .223’s will warn against using 5.56mm ammo in a rifle chambered for .223.
p.s. It is hard to imagine a .223 round (with what? maybe a 2,800 fps muzzle velocity) remaining inside the head if shot directly, barring the bullet being something like a Glazer Safety slug type round...which very few .223’s are....fmj most commonly.
A ricochet where the initial impact is the floor and the person is 2ndary impact, is more likely. You might read how Israel used low power (subsonic) .22 rimfire rounds in govt usage for wet work, in suppressed .22rf pistols, for reason of quiet and for head shots, the round stayed inside the skull.
.223 bullets mostly are sharply pointed penetrators of small frontal area which allows greater penetration travel, due to less resistance than an equivalent ft/lbs M.E. bullet of larger caliber would have.
The recommendation against using 5.56mm in a rifle chambered for .223 has to do with different headspace, not intrinsic power of the round.
The 5.56mm is slightly longer and when fired in a .223 rifle the bolt may not fully lock.