In my experience, I was already armed with both a shouldered long-gun, and a shouldered melee weapon.
Find a green, steel guard tower of approximately one square meter floor dimensions; such as those around military bases, or near Zelenogorsk or Drozhino.
Pick up a new shoulderable weapon to replace the one already on your back.
Shouldering animation should end somewhere outside the tower, resulting in a fall.
I also did a crouch-walk around the loot pile in the center of the tower, trying to inspect its contents, before picking up the pickaxe to see items underneath it; this may have contributed to my position being confused by the animation.