Despite all the interesting information and opinion these posts have given; I would still be interested in feedback from someone with operational experience.
Shooting bullets into ballistic gelatin does not replicate what happens inside the opposition. I can qualify this by the example of the 10mm FBI rounds, which they believed were good because they looked good when fired into gelatin. Unfortunately they didn't perform well when fired into bank robbers.
I know that if you put any bullet, even a .22, in the right place then it's goodnight vienna. Interestingly in the 50s & 60s, Mossad's prefered weapon for despatching war criminals and middle eastern terrorists, both groups of people they disliked, was a .22 LR Beretta. Naturally they would have head shot their enemy and it would be all over.
My experience of the 5.56mm when first issued in Northern Ireland was that the terrorists didn't drop when you shot them (this was with the L2A2 5.56mm rounds that were correctly pointed out to have a thicker jacket). Because of the fear of prosecution for murder in Northern Ireland, we were often reluctant to pump numerous bullets into terrorists; but there are cited cases of major shootouts where soldiers put five or six into a terrorist and they crawled away to bleed out.
I think 5.56mm is an inherently good round, especially when fired through a 20-inch barrel rifle. I don't think it reaches its full potential in an M4, despite this I do like the M4 but one has to appreciate its limitations.
I left the Army 8 years ago, so my first hand experience is limited to pre-2005 operations. I know that in the Falklands the enemy dropped like a stone when they were hit by a 7.62mm; the same happened in Northern Ireland, until we switched over to 5.56mm in 1987.
The first kill with an SA80 in Northern Ireland was over hyped by the tabloid media; they claimed a 500metre night time one shot kill in Belfast! What really happened was more comedy than good shooting!
The terrorist was coming out of a safe house with an AR-18; he almost immediately encountered an Infantry soldier coming around the street corner armed with an SA80. The soldier's SA80 was set to 'automatic' (no idea how or why on that one), but at a range of under 10 metres they saw each other and the soldier put half the 30-round mag into the terrorist! Whilst it was a 'good shoot', the soldier got a collosal bollocking for shooting him so much on automatic...but then carried on his tour without penalty!