First hand knowledge preferred.Was the 5.56 a better manstopper with the old 55gr 1-12 twist? Or as they increased bullet weight and twist a better fight stopper? IMO it just increased penetration.Lots of stories of bad guys taking several heavy ball rounds to the chest and continuing to return fire for quite some time.
Define manstopper.
Define fight stopper.
Define quite some time.
55 gr with 1-12 twist is not an old idea.
The 5.56 round adopted for common service by Nato countries in the 60s-80s was generally a 64 gr bullet in a 20" barrel. The intended target was a soldier clad in soft armour and steel helmet, so penetration was a significant factor in performance.
The trend in the last 25 years has been towards progressively shorter barrels as combat moved away from the jungles of korea and vietnam and towards the dense urban environments of somalia, and iraq.
Anecdotal evidence from soldiers regarding failure of the round to immediately incapacitate are ipso facto unscientific and have not generally been supported by lab research.
In practice, the ability for a one shot stop is highly dependant on shot placement. Variations of milimeters can be the difference between passing between ribs and causing significant organ damage or deflecting off the bone amd causing grave, but less immediately incapacitating trauma.
Any average loss in probability of instant murder/death/kill has to be weighed against the fact that shorter barrels usually mean more agility in close quarters. The smaller cartridge of m223 was chosen over .308 for a variety of factors including being able to carry more ammunition.
The fact that we are hearing so many stories about people who survived gun fights with these observations tells you they are still winning the gun fight.
The question isnt how many rounds does it take to kill a drug adled insurgent, but how many good guys died because the round couldnt instant/murder death kill, and if thats an unacceptable number, is there a better firearm, barrel, twist, cartridge bullet weight that provide a net reduction in that number.
After 40 years of service and study the jury is still out.
At the end of the day, shot placement matters. The first person who scores good hits on target typically wins the gun fight.