Our investigator is urged to go very cautiously here.
There are two MIL STD rounds for 5.56x45mm: M193 and M855 (now M855A). M193 fired a 55gr bullet with a gilding metal jacket and a lead core to a muzzle velocity of 3280 ft/sec. It is no longer standard (but equivalents are widely available on the commercial market). M855 has a gilding metal jacket over a hardened penetrator with lead filler (original configuration) and fires to a muzzle velocity of 3100 ft/sec.
Some decades back, military firearm authority Peter J. Kokalis stated in print that the M193 loading was more lethal than either 30M1 (US 30-06 WWII: 153gr jacketed, 2750 ft/sec) or 7.62 NATO (147gr jacketed, 2740 ft/sec), within ranges of 100m. Out to that distance, the M193 projectile retained sufficient velocity to fragment on hitting a human body (no, he did not say where). Beyond that, it punched through - like the larger, more stable 30 cal bullets did at lesser ranges. By contrast, M855 was designed to give longer effective range and afford better penetration at long ranges - it will out-penetrate that 30M1 bullet at 850m.
I’ve witnessed 22 cal soft point bullets punch clean through the web (support portion) of a railroad rail. Fired from a 22-250 at some 3500 ft/sec; range was under 50 ft. We did not recover any spent bullets.
Penetration is thus probabilistic (not much help for our investigator, I know), and highly dependent on impact velocity, tissue (bone, skin, muscle etc) density, angle of incidence, etc. Humans vary a lot: not even a side-by-side comparison of bullet wounds can tell us much.
One hears that the mere discharge of a firearm in the general direction of another person constitutes intent. Not sure how well this legal ploy works.
5.56mm MIL STD ammunition is identical in exterior dimensions to commercial 223 Remington. The warnings from manufacturers not to fire 5.56mm in a 223 stem from the difference between military and commercial chamber dimensions. Military chambers are cut larger, chiefly to improve feed reliability in full-auto arms. The leade is also longer. Military chambers also need more generous tolerances, to handle ammunition of allied origin. Some commercial chambers can be very tight; this is usually done to enhance accuracy (in bolt action arms) but ups pressure. The warning applies chiefly to M855, which can produce higher mean pressures.
I think everything you said generally supports my hypothesis. Nothing in it suggests that the round you discussed would break apart in a human skull at 10 FEET! And surely not without enormous pressures within the brain cavity, resulting in blood from the orifices in the head.
The victim only bled from the entry wound with some slight bleeding from the left nostril.
Can’t make that square with any shot from any AR round from 10 feet. Excluding a .22LR conversion of some type.