You could add an END reserve based on mental strength, the more bullets you fire, the more mental strength you use. I would allow an EGO roll instead of a Fast Draw roll to reload as a half-phase. In a sense, I would say that you reload quickly by pushing your mind's strength to reload.
I would allow you to build two guns with Autofire: 2 shots and then make combined attack when using both. Perhaps your GM would make it a Multiple Attack, which in that case you may want to take Rapid Autofire and Two-Weapon Fighting.