1. bare hands
2. select a weapon on your shoulder: ex Winchester with scope, press "1" to put in hands
3. use right mouse button to scope in
4. weapon goes back on shoulder.

Once you do this once it appears to stop until you try another weapon in your inventory - such as a hand gun in slot "3".

I did not try it for an axe or other items on my hot bar, but i would say given it is consistent, it might be for all hot bar items.