I think it's taped to the bipod, it wouldn't last through the first burst taped to the barrel, specially right there at the gas.
I had an M60 issued in Canungra that had a five digit number, no doubt it was an RVN survivor. It had a hard time with the plastic blanks, but that's no measure. The three water bottles was not unusual, you need water in you, the stuff in the jungle can't be trusted. The weight doesn't matter. We too would remove slings when noise mattered most. You're supposed to have the weapon in hand anyway... Their rifle section had an M60, two M16s for the lead scout and the section commander and the rest were SLR. I think you'd find the man foreground has three water bottles on his belt and one on the ruck that you see. More is better. They get lighter as the day goes on...