I use a .243 to hunt deer, so am familiar with small rounds and wounds. However the 5.56 is underpowered for service round IMO. I killed a deer at 425(measured) yards in Colorado several years back(2000 I believe, Unit 21) and hit it in the heart. It ran about 20 yards and fell over dead. The load was a 100 grn. Hornady and I forget what powder. The gun was a Ruger M77 with a 12 power Redfield variable scope and sighted in for 200 yards. One shot did it, BUT when I am shooting to save my life I want the biggest I can handle:)
The whole idea behind 5.56 was that the soldier could carry more ammo. In Nam I have read that there was something like 5 million rounds expended for every enemy confirmed KIA. Seems way high to me but I witnessed one fire fight where a company of Rangers expended well over 10,000 rounds and could not find any bodies (Charlie used to drag them off). In this case there wasn't even any blood trails.
I was hunting deer outside Winner, South Dakota last fall and popped a doe at well over 300 yards with my Sako 7mm Rem Mag. The wind was stronger than my estimate and I only hit her in the back of the left shoulder from the back. She limped off and I found her later behind a hay roll. The bullet had lodged under the shoulder blade. Perfectly expanded and stuck in the bone. My bad poor shot. But venison in the freezer.