It's actually a very early gun and only the main character has one. The developers have said that it takes a lot of time to aim it for an accurate shot so I'm guessing that to use it to assassinate someone you have to stand around aiming forever, while totally open to someone walking along and spotting you, and that it probably misses from long range, meaning that you have to be close enough that the target or a bodyguard might spot you and ruin the whole attempt. Also, cool people don't use guns, they sneak up and use the hidden blade.