in order to make a item spin in somebody's hand, first you need to know how to make someone hold an item, which is done by clicking F on the humans arm you choose to hold the selected item
next you need to have a human/ robot hold the item and nail the said hand with the weapon

the next thing to do is when you want them to spin the gun

make the human drop the item (dont remove the nail) and boom! they spin gun and commit high noon moment :D