Maybe this is the Nicholas' AA problem?

[x2_Weekends]

 

I've never found aiming the handle of the gun at the aircraft very effective for shooting down planes.  How about you?  (That is the handle, isn't it?)

[Final_Spark]

No, not the handle. 50 cals always have two small handles on the back. If you look closely, you'll see the hole for the end of the barrel.

 

For what's pointing down? Not too sure. Perhaps something to do with the water cooling.

[_Platform]

Unless it's one of those new-fangled bent barrels that sends the rounds down directly into your own ship.

[Mizzerys_Fate]

It's a barrel cover to prevent corrosion from sea spray entering end.