If you're playing in a 2d6 game there is already a rocket pistol -the snub pistol. Snub pistols have their fans because I've already seen rules for snub machine guns and snub carbines. Hell, I wrote a couple of them myself.
Following are some system agnostic ideas for rocket pistols.
First they're stupid simple to make (the ammunition is another matter). The rockets have little kick (in the barrel at least). They are caseless which is important if you're worried about brass fouling some machinery or controls or a hot spent cartridge landing inside your shirt. I've had this happen and I swear the little bastards look for any opening and hurt like a sonuva mother.
Unlike most caseless weapons rocket pistols do not suffer from heat buildup. Cased rounds carry a lot of heat away from their weapon when they are ejected. Caseless ammunition leaves the heat behind and it can lead to burnt fingers or even co0king off when a weapon fires off an entire clip despite your best efforts.
They have no kick worth mentioning. This is a bit of relief for shoot outs in zero gravity. Despite what some movies will have you believe firing a handgun won't send you spinning crazily. It will require you to adjust your aim and grip on whatever you're hanging onto.
The lack of a kick could make it easy for careers (or classes) with weapons restrictions to use. You might want to allow anyone to grab a rocket pistol and start blasting. That's pretty big in terms of training times and could make them ideal for militia or rebels. In Armageddon: 2419, Buck grabs one and proves himself a dead eye with it after a few shots.
They're stealthy. A phaser and most energy weapons will give away your position. Most firearms are very difficult to suppress (forget about actually silencing them). The rockets fire with a small puff and hiss. People who are shooting with surprise or from hiding might be able to get a punch of cheap shots in before their enemies realize where the fire is coming from or even what is happening.
Rocket pistols can also be an easy way to tell the good guys from the bad guys. In combat this is not as easy as you might hope. Now you have a way to tell if those guys are on your side or not. Just fire a rocket over their heads. If they hose you with rocket fire, they're on your side. If they disintegrate you they're on the other side.