Dave Asks: What is this?

That is a M-2 .50 caliber machine gun with a blank firing attachment mounted on a M113 family personnel carrier, probably rolling around Ft. Irwin, CA.

The M-2 is blow-back operated: it's mechanism depends on the backwards force of the bullet being ejected from the barrel. Since a blank is just a small powder charge, it doesn't have the same force as the full powder charge and bullet, so the blank firing attachment covers up enough of the barrel muzzle for the blank to produce the force to operate the mechanism. It is separate from the barrel because the barrel of the M-2 moves backward (hence, "blow-back operated"). It also includes a metal piece that goes into the feed tray of the weapon, which prevents real bullets from entering the weapon, a real disaster if you have that BFA mounted.

The operation of the M-2 is similar to how pistols operate, only on a much larger scale. What is remarkable about the M-2 is that is is a highly accurate weapon, despite it's barrel moving around quite a bit. I think it is probably the largest blow-back operated automatic weapon in existence or in common usage. Some other large weapons, such as the main gun of tanks or howitzers, also use blow-back force. However, the mechanism of cannon typically is confined to shock absorption and breech opening (and the breech ejects the spent cartridge or base case). They are not truly automatic in that people load the next round in them, or are automatic on an electro-mechanical basis, in that the blow-back force does not also load the round, but a mechanical contraption powered in some other way loads the next round.

For the vehicle ID, I was confused at first by the soldier with the binoculars on the left. He is actually sitting on top of the vehicle and the metal in front of him is the characteristic upwards facing engine exhaust of the 113. The vented circular object in the center foreground is the horn.

To the far right of the picture frame, you will notice two upright and curved at the top tubes of silver metal. This is part of the cage of the "whoopie light," a signal light mounted to vehicles while using MILES gear, a militarized version of laser tag. On the M-2, just above the base of the barrel, you can kind of see the laser, which is a little metal box mounted on top of the weapon, that shoots each time it feels the shock of a blank being fired.

Since this is commonly used at large scale war games at the National Training Center (NTC) at Ft. Irwin, and the mountains in the background seem characteristic of the mountains and valleys Mojave desert there, I figured that this was taken at the NTC. The soldier to the right is wearing a CVC helmet, (Combat Vehicle Crewman helmet) and the soldier to the left is wearing a standard kevlar helmet.

In my former unit they could be mortar guys or mechanics. I think they are mechanics because they look a little bit messy (sleeves rolled up, on the guy to the left, neither he nor the track commander to the right is wearing a BDU top, despite obviously in an action pose for a picture. The binoculars look to be of an older variety, which opens up the possibility that they are cavalry scouts or mechanized infantry, since it would be before the Bradley Fighting Vehicle was fielded. An additional possibility is that they are national guard, in which the messiness and dumb looking action pose would fit as well.