Actually dude, I'm gonna contradict you and say it is possible, although it will need some alterations at the modelling level.
You will need one animation with a firespot bone over the rocket pack, and one animation with the firespot bone over the MG.
Then you need to create a rocket launcher, but make the Vweap file a tiny (almost invisible) cube.
Set it's weapon as the almost invisible rocket launcher.
In it's script when it attacks it will randomise between rocket attacks and MG attacks.
If it's shooting the rocket launcher, you play the animation with the firespot over the rocket launcher, run the decal and "useweapon". If it's firing the machine gun you run the animation with the firespot over the machinegun, run the decal, and decrease the player's health (just don't use weapon).
There. 2 different weapons, 1 model, no source mod needed.
You only issue will be FPSC's hard coded animations on animated characters with weapons. As you will need two attack animations, you'll have to sacrafice one of its other animations, (like strafe or crouch)
