The M60 machine gun. A gun developed from the lessons of WWII, it is a gas operated, air cooled, belt-fed weapon that fires the same standard 7.62X51 NATO ammunition as the M-14. Here's a history of the development and use. It's an iconic weapon from the Vietnam era, one of the statues at The Wall is carrying one.
Here's a wallpaper sized image. Click to biggify.
newrebeluniv has this one checked as his grail gun. There are some transferable ones available, running $30,000 and up. I'm guessing his second purchase would be some truckloads of belted ammo to go with it, and a fair sized pile of spare parts.
Update: One man's grail gun is another man's day at the range. Murphy's Law has one. Here are his posts on his M-60.