There is a locking pin that locks the machine gun in the horizontal position. When removed, the machine gun will point up (the receiver end is heavier than the barrel end). A similar pin locks the machine gun in a position tangent to the hatch opening. That way the machine gun doesn't swing around on its own while the vehicle is in motion. Same basic controls still used for the M113s, M88s and just about any armored vehicle (less tanks) in use by the US Army today.

I kind of figured that the "unlocked" position was the one with the muzzle pointing into the sky. I just wasn't sure if the horizontal position was a mistake - one of those instinctive things you do on a model without realizing that it is completely wrong. You know, kind of like how I used to paint all the landing gear on my aircraft aluminum, before I realized most were painted white. Anyway, I've started to mounted a few of the machiine guns on my tanks at different angles - just to mix things up a little.

In most of my reading, I only recall a few instances where the MG was used against aircraft. I know that the movies like this action-packed circumstance, but my feeling is that the most common use of the AA MG was in the ground support mode. For the reasons discussed above the MG would not be particularly effective, and if you consider trees and terrain I would think that the opportunity to shoot at aircraft would be limited. While the US Army has not completely given up the fantasy of tanks shooting down airplanes, we certainly don't train to do it in most units, and only the "duck hunters" waste any ammo on the effort.

Most of the photos I have seen have the MG in the horizontal position. Since the MG could not be pinned with the muzzle in the air, it would bounce and bang around while the tank was moving, which is an event of concern with a 100lb+ MG. So being locked in the horizontal position makes sense as the most common situation.