A gov't profile M4 barrel will get hot enough to begin cook-offs after about 180-200 rounds fired consecutively(full auto).
In our tests, the M4 barrel was between 900 and 1000 degrees F, 3" forward of the chamber, after 180-200 rounds in full-auto.
Cook-offs began, and in both tests there were 2 cook-offs between 24 seconds and 85 seconds after ceasing fire.
A standard M4 barrel in 155 degree F ambient temps had 10 cook-offs after 200 rounds fired continuously, and was still cooking-off after 2 minutes of cooling time.
These were GI M4 barrels, chrome-lined.
If you bump-fire 5 mags, you will be getting very close to cook-off temps. On a hot day in the desert, you may even get some cook-offs after 5 mags rapid-fired.
Heavy barrels fared much better, going from 220 rounds to 260 rounds before reaching cook-off temps.
These were laboratory condition tests, with heat sensors positioned every inch along the barrels.
Tom Lyons
MGI