You can't guide someone in finger posing, just use the tool on the hand and hold the C key to be able to edit the fingers and still be able to see them and practice.
Good finger posing takes a LOT of time, but it's good enough if you make it look good in the screenshot, it doesn't have to look right from all angles.... Hell, you could just do an optical illusion to make it seem like he's holding the gun but isn't and it'd work fine
