It is not a good start for a story if the protagonists kill each-other on sight when they first meet. Unfortunately, my protagonists are very likely to do just that. They do need to defend themselves against nature, rivals and the police while they carry on their not quite legal activities. But there is a simple solution: Give them non-lethal weapons.
The setting is a futuristic space opera, with a bit of a western flavour. Think grease, rust and iron. The universe is not a well policed place, plus the starting place of the story is a planet where nature is a threat. People usually carry a weapon with them. For plot reasons, I don't want the standard weapon to be lethal. However it needs to seriously incapacitate the target for at the very least a few minutes, up to hours. It can paralyse, knock out, put to sleep, whatever; it's up to you, as long as your opponent is instantly not a danger any more. The effects must wear off by themselves in minutes/hours.
The standard weapon also needs to work against the various intelligent species you can encounter in this universe. You can assume that the vast majority of them share the same basic structure (carbon-based, breathe oxygen...) and range from the size of a big cat to the size of a rhino, the average being between human and horse. This is a wide range, so I'm perfectly okay with the weapon being sometimes deadly if not "dosed" appropriately.
Fashion constraint: the standard weapon must look more or less like a gun, use bullets and make a nice gun sound. A valid "bullet" for this question is some sort of charge you have to insert, which get ejected through the shell and doesn't stay connected to the gun; and that you can carry in a limited quantity.
Bonus points: It makes an open wound, even a tiny one. You can sometimes kill your opponent (hit a specific area, target too small?). Should you accidentally deliver a killing bullet, there is a remedy/a procedure you can do to save your target.
Thank you for your time.
EDIT : Several answers give interesting & complementary solutions. I accepted the one which gave a direct solution to "how does a non lethal gun could work". Other useful answers :
immobilize opponents with sticky-expanding-sturdy-foam I like how this one works on every creature ever
Thank you all for all your interesting propositions !