[RetiredNavy]

See what you started Dead,

the problem has NOTHING to do with the thickness of the barrel. Please see the post above for the correct information.

[Dead Corpse]

Design of the barrel is everything. From how much chamber pressure it can safely hold, to the final numbers on the external ballistics, to how much heat it'll dissipate under sustained fire.

Metallurgy, machining tolerance, heat treating, etc... all are parts of it.

Again, it all comes down to how much pressure a given design can deal with. 5.56 chambered pencil barrels will have the looser tolerances. They also lack accuracy because of it. The same pencil profile in .223 match chambering would indeed come apart under sustained 5.56 diet. The same .223 chambering with better steel and a bull barrel profile can eat 5.56 all day long, even if you end up ironing a few case heads.