I mean, modular connector, then screw a weapon on.
Instead of the limb. There aren't listed with an Essence cost. Especially things you can't normally mount, like, say, an AR.
AH! You just want to strap a gun to a stump? Yeah, you can do that, but keep in mind, you aren't going to aim worth a damn. It isn't a cybergun, and it ain't a cyberlimb, so it's not designed to work with your nervous system or to allow you to point it as easily as you point your arm or finger. (Seriously. Close your eyes right now, then point to the light switch in your room. Open your eyes, and you should be pretty close. Human bodies are neat that way.) Now balance a pen or pencil on your trapezeus (the muscle between your shoulder and neck) and try the same eyes-closed-aim trick. It's VERY unlikely to be on target.
Recoil might tear it off, depending on how big the thing is, and, in general, you're not going to be hitting a dang thing. It'd be about as useful as putting on a baby-holder to your chest and duct-taping a gun to the front of that.
Won't cost you any extra essence, but you're looking at an accuracy of 1 if you're lucky.
In short? Bad idea, but if your GM wants to let you give it a go, feel free.
(You'd be much better off getting a modular connector, taking a normal cyber arm, then taking an extra arm with whatever cybergun you want built into it.)