A great question! I love it.
So…Here I go…

On the typical mounts of US tanks, halftracks etc, the gun itself is mounted on a cradle that holds the gun in two locations (front and rear) with two locking pins. The cradle is then held within a hinged mounting roughly in the middle of the length of the gun (roughly along the balance point of the gun to make it lighter to move around) the cradle also carries the ammunition box mount, allowing it to move vertically with the gun and not twist the ammunition belt.

The cradle itself has it’s own locking pin that, indeed, locks in in the horizonal position, and only horizontal! The mounting point for the cradle and mount assembly has it’s own traverse lock but that isn’t in a fixed position, so the gun can be locked in traverse at any point in its 360 degree motion.

(this image shows the m2 pint mount with cradle and locking pins in place. the lowest little handle with the chain hanging fro mit is the horizontal locking pin.)

When the gun (in this case the .50 m2) is not in use it can be stowed externally on the vehicle after being removed form the pintle mount and placed within its own canvas bag. On some US used Sherman tanks you’ll see the stowage points on the turret rear. Alternatively some Shermans had turret roof “travel locks” which is a bracket that rises up to clamp the barrel of the gun to face forward on the turret roof and keep it clear frm the commanders and loaders hatches.

As an aside, all tanks and vehicles with pintle mounts will usually carry the ground mounts (usually tripods) for all mounted machine guns.
I hope this helps! Most pintle mounts act and work similarly (German pintle mounts are weird because German)

Not sure how realistic this would be during war-time, but in these images show pintle mounted guns locked into position with a canvas (?) cover.

AFAIK a few vehicles had the guns dismounted, but I don’t have any images of them, because I was more focused on watching the parade.

Also for dusty road moves, covers are essential.

Most pintle mounts act and work similarly (German pintle mounts are weird because German)

I heard that! @johnlyons

For what it’s worth: in my time in the Bundeswehr if a vehicle had a hatch for a gunner there was a mounting point for the MG3 where the bi-pod legs would go in normal operations. Most of the time, when the hatch was closed the MG3 was stored inside. Only when we did things like a TacEval it would remain outside and there is a bracket to fix the back end of the MG3. And a cover that goes over everything for keeping it nice and clean. That’s how we had it on the Mercedes Benz 2t MilGl, MB 1017 TmilGL and the bigger 5t, 7t, and 15t MilGl by MAN.

The Bundeswehr version of the ILTIS was fordable with a solid body pan

And it was a fun drive in rough terrain. 4 wheel drive with a turbo diesel engine, large diameter wheels. That thing get’s anywhere.

Just don’t go above 100km/h on the Autobahn. The wheels start screaming and the foldable top is no fun in winter XD

On the M48 I have seen two hatch types for the commanders position. One being :

The other being this monstrosity…

The first one showing that big pintle mount, which, to my eyes, looks to be locked horizontally. I know on the older WW2 era mount that the gun cannot be locked in any other position but horizontal. And this one on the M48 would, in my opinion, be the same. As others have pointed out it keeps the gun where it’s “less likely” to catch on low hanging things and, to a degree, offers the commander some protection from traps like wire strung at head height for tank commanders between building and the like. (yes, that was a concern, and yes some vehicles had addons to protect troops form that threat, see below!)

(The long, angled peice of metal on this Ferret’s turret front was for cutting wires and obstructions before they removed the commanders ability to think)

Externally mounted guns on the cupola of a vehicle, as others have said, normally needed to be manned from outside the tank. Partly why you rarely, if ever, see a .50 cal mounted on top of british Shermans past, maybe, the desert campagin as they decided it wasn’t worth getting out of the tank to operate.
Also of note is that cupolas generally could rotate themselves

This Sherman hatch assemylu can rotate 360 degrees with that gun mount able to rotate itself. So the cupola can be rotate to have the gun mount out of the way and locked in that position. Both the gun mount itself and the cupola have independent locks to achieve this.

It makes sense to me that the M48 commanders cupola can do the same. But maybe I’m applying too much logic to something built by the DoD.. 😉