In our barrel torture testing at MGI, I've seen handguards actually on fire!
I concur that the gas tube remains intact beyond the failure of the barrels.
Cookoffs occur much earlier than failure, and effectively disable the weapon from use temporarily.
M4 barrels will typically start cooking-off before 200 rounds are fired in full auto.
Much better barrels are required for full-auto use in a squad auto type role for the M16.
Barrel cooling technologies are also needed, so we can have lightweight barrels which will withstand extended full-auto firing.
We are working on it, and have some outstanding prototypes.
In the meantime, our QCB system allows quick change of barrels when hot or cold.
We are developing a modular AR platform to allow configurations from entry, to duty, to sniper, to squad auto LMG, all on the same upper and lower receivers, with quick change barrels, modular magwells, and even a belt-fed option, and open-bolt/closed bolt fire control, and different caliber usages. The MGI QCB upper receiver system is the first part that is available to the public. The modular lower receiver is next.