Read this
http://www.imageseek.com/m1a/M14_Preservation_Lubrication.pdf
In the cookoff test, the U. S. Army determined...
During development of the Mk 14 Mod 0, the Naval Surface Warfare Center (Crane, IN)
found that cookoff could occur in as little as 150 rounds of continuous semi-automatic fire
with a wood stock M14 on a hot sunny day. NSWC, in later testing, obtained the following temperature readings on a Mk 14 rifle fitted with a Smith Enterprise, Inc. M14DC sound
suppressor and a third generation Sage International, Ltd. M14 EBR stock:
Mk 14 rifle fired in semi-automatic mode for 160 rounds at a rate of one per second then
allowed to cool -
chamber temperature - 178 degrees Fahrenheit
gas cylinder temperature - 496 degrees Fahrenheit
sound suppressor temperature - 734 degrees Fahrenheit
cookoff - no cookoff after ten minutes
Mk 14 rifle fired in automatic mode for 200 rounds in twenty round bursts -
chamber temperature - 250 degrees Fahrenheit
gas cylinder temperature - 525 degrees Fahrenheit
sound suppressor temperature - 1,107 degrees Fahrenheit
cookoff - cookoff occurred at sixty seconds