Its the cycle time. If you can cycle a bolt as fast as an auto for example, the caswes from the bolt would be as warm. Lets see that would be around 800 for an ar, around 600 for an Uzi etc. Its not possible to get to the bolt fast enough. This feature is used in select fire weapons as an advantage. The ejected brass acts as a "radiator" to pull heat off the weapon before it can be absorbed. Gas opertated weapons are designed with escape ports to remove this gas after the bolt assemblies have moved a specific distance so as not to have gas left in the barrel.
