What are the names of control options that allow you to change modes and fire the railgun? I can't find any information on this online. And also in the in-game controls menu, there is only one button under Overloaded and that controls something about the armor and doesn't work with the railgun

right click shoots, and some combination of something+scrollwheel to change power draw

does it need some sort of ammunition?

Forge energy units, can be filled in item interface or most modded charging blocks

Closing the issue, assuming your questions have been answered.

Please follow the issue format if you have questions/issues in the future, as mod/minecraft versions are sometimes relevant when a mod exists for several versions of minecraft and has changed in some ways over the years.