Exit wounds.
The .223 comes in several cartridge sizes and powder loads. There is a hunting cartridge called the .223 Bee that is the fastest velocity in the world of small arms ammunition. Over 5,00 feet per second muzzle velocity. It is a necked down 30.06 cartridge case with a .22 cal bullet. It has such a high velocity that it tends to wobble in flight and has been criticized for this reason. Up to three hundred yards it has practically a flat trajectory but over that distance is very unreliable. The police investigators should be able to determine the type of .223 used if they can find the distance from the victims.
This is a long barreled hunting rifle and scope if the shots were as accurate as the authorities maintain. The M-16 does not have a long enough barrel to project the accuracy they are talking about. Two hundred yards is the maximum range for a sure shot. At that distance the velocity of the bullet causes the same knockdown power of the 30.06 in spite of the small bullet size.