The power switch is found on the top of the Nerf gun near the stock end. The only way to use the Nerf gun is to turn the switch into the "on" position, it does not have a manual firing feature.
Unfortunately no, you cannot exchange the stock end of the Swarmfire and use it on other devices by Hasbro. This Stock end was designed only for the Swarmfire and will not securely fit with other devices.