Just a 1/6 gun i made at work.
The idea was a dual one(Either to be used against Hulk or Superman), but for use against Superman, here's the thinking: The phial on the rear contains a liquified kryptonite solution. There's an ammo magazine on the top. Just before each round is chambered, a small amount of the liquified kryptonite coats the chamber, thus coating the round before it's fired. The catalyst is firing the round, which hardens the solution around the projectile itself.
So now there's a kryptonite covered bullet streaking towards Superman.
Superman, being totally invulnerable to everything, wouldn't even waste his time to try and avoid it...
And when it goes through his suit, and into him and he feels the pain of being shot...
He realises(Too late) that Bruce will be a stronger opponent than he thought.