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Here’s a writing from someone I consider more than an expert on such things: http://www.ballisticstudies.com/Knowledgebase/.223+Remington.html

And an excerpt: From the very outset of its adoption the M16 was plagued with troubles. Stoner had designed the original AR 15 with a very slow barrel twist rate of 1:14 which was literally a doubled edged sword. By using a slow twist barrel the 55 grain bullet was only just stable in flight, producing a small degree of yaw. On impact the bullet would immediately tumble and render a wide, incapacitating wound. This was initially considered a brilliant design premise but some rifles produced too much yaw and were very inaccurate at longer ranges. McNamara ordered that the twist rate be changed to 1:12 before final adoption of the rifle in 1964. This cured longer range accuracy problems but completely destroyed the stopping power of the 55 grain bullet which now poked needle holes through its victims. Nobody questioned the potential consequences of this move and ignorant of the facts, Ordnance brass continued to believe and promote the M193 as a highly effective cartridge

Also on the site is wound research done on many different
animals with many different cartridges. 5.56 were examined on wild goats mostly.