Morphs may not be the best way to move parts. Making parts move on prop, parent the moving parts to the gun and you can use the joint editor to adjust the parts center and endpoints. Rename the trans/rot dials to say something like open/close or forward/backward, up/down or even cocked/uncocked. Set thier limits and hide the dials you don't need.

There is also the Phibuild and Sams3D had a very good tutorial on how to use it. Don't know if they still do but they are pretty good at helping if asked.