As the M60 was laying down fire it started getting red hot, how many rounds can your fire before destroying the barrel?

What did people do in Vietnam if they had to fight for hours?

I think 100 rpm barrel changes every 10 minutes
200 rpm barrel change every minute. (IIRC)

Barrels will droop if you get'em to hot I would guess you would need a very long belt 500-1000 to really do it. The M-60 is supposed to be fired in 6-9 round bursts not continuously. I think in real combat it would be tougher to keep enough ammo close by to really melt a barrel if it was foot infantry.  

Most Inf. in Vietnam split up the support ammo amongst the squad, 7.62 belt, mortar rounds, LAW rockets, M-79 rounds. Plus they carried a ton of .223. If you were the 60 gunner and you ran out of ammo your ass would be grass if you survived the fire-fight. The rest of the squad/platoon would want to know exactly why you burned through all the ammo.....

I believe that was a SAW in Proof of Life, not a '60.