I don't know this to be fact but I would guess the Rangers in Somaliland were using the newer heavy bullets. They give better ballistics downrange and will penetrate extremely well, in fact I once did a test between standard .308 and the heavy .223 and surprisingly, the .223 penetrated better at long range.
I would bet tho that these bullets just go right through without doing as much damage as the 55 grain bullets.
I doubt if any of the .223 will stop as well as a 30-06 or .308 but they still can be very effective.
I asked a gunsmith who makes quite a few AR's for local SWAT and police about Somalia. His explanation was that current M16 issue barrel has a 1:7 twist while the older military barrels were, I think he said 1:14. This change was made to stabalize the heavier bullets. It also makes the lighter bullets much more stable and prevents the tumbling effect. He also added that many police often have him rebarrel their M16s to the older twist.