As the title suggests.

When I'm carrying a mosin on my back and shotgun in my hands, I just drop the shotgun at the bottom part of the ladder, climb up the ladder and when I reach the top, the shotgun is in my "vicinity" and can be picked up from like 20 meters below me.

There doesn't seem to be a plausibility check that determines whether the gun is vertically in my vicinity, only horizontally.

Probably works for similar ladder scenarios, haven't tried though