Modern assault rifles are not designed for automatic fire so if the the issue occurs when firing in controlled manner then i would say the gun must have design flaw.
In particular if its matter of 100 rounds but yeah it can get very hot.
MG's with their relatively heavy design usually have changeable barrels but still operators are toughed to shoot in relatively controlled way as well while switching barrels under combat situations.
Suppressive fire is not necessary about emptying all ammo supply in 5 minutes.
When i got to shoot MG i remember changing barrels in range of 250-500 bullets and sometimes colling the gun with drinking water to keep it going.
That is when intensivly shooting in long or short burst.
Or sniping long range with the same 7.62 Belgian MAG.
