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.
good point....shape of entry hole would also be important. Shot direct to the body would be a round hole...there would be no indication of anything but a round hole....bullet would not be tumbling.
anyway, good luck. the defendant can be garbage, but the issue needs be guilt. low quality types are probably at increased risk of safe and prudent firearm handling. Macho male attitudes encourage unsafe handling.