cuervo

Here is the semi-auto rate of fire information for the M14-type rifle:
(from FM 23-8)
1 min --- 40 rounds per minute
2 min --- 40 rpm
5 min --- 30 rpm
10 min --- 20 rpm
15 min --- 20 rpm
20 min --- 20 rpm
30 min or more --- 15 rpm

So, in 10 minutes, I could shoot 200 rounds without damaging the rifle. At the end of that 10 minutes, the barrel could easily give you a nasty burn. The accuracy would suffer, but it would still be within specifications. If you were shooting for match purposes, the accuracy would be off more than acceptable. Unless you really, really, heated up a barrel, or did something to it while it was really hot, you won't damage the barrel and it will retun to its "cold" position, or close to it.

There are several ways to reduce heating and/or maintain accuracy. You could flute the barrel, giving you more surface area to disipate heat. You could free-float the barrel, so that nothing is touching it as you shoot. You could cryo-treat the barrel, which removes internal stresses in the metal, so the barrel won't deflect as much when it heats. The effectiveness and cost effectiveness of all of these could be argued.