To add to the challenge, balance a coin or a spent casing on top of the barrel and try to drop the hammer without disturbing the coin/casing.
Obviously, if you use the casing, you can't use the sights, so you'll have to do that separately. And if you use the coin on a gun with a flat top (like a Glock), it won't be challenging at all.
"An armed society is a polite society." — Robert A. Heinlein
"After a shooting spree, they always want to take the guns away from the people who didn't do it. I sure as hell wouldn't want to live in a society where the only people allowed guns are the police and the military."
— William S. Burroughs