Google is your friend. There’s more than enough information to get an idea of how they work, and a number of different ways to make one.
There is very little comprehensive information comparing superguns because there are only a couple ‘manufactured’, and MAS is the only one I know of manufactured in the US. They are very niche items, and the majority of them are custom made jobs, such as the awesome ones MKL makes.
There is no off the shelf converter you can use except the PSX->Neogeo adapter, and even it is limited to the four face buttons, d-pad, and start/select. Even then, you’d have to verify that the controller connector on the SG matched the Neogeo pinout. Almost all custom made SG’s do, but most manufactured ones don’t. Getting something that would allow for 6 play buttons or more would require a custom made adapter. A few people can make them, such as laugh or myself, but its a custom electronics gig so it could be spendy. DreamTR has a thread in the Trading Outlet asking for one to be made, and N-Ken has a thread on neo-geo.com asking for the same thing. BTW, having an xbox adapter for SG use is close to out of the question. PSX is doable, Saturn is fairly easy, and Neo-geo is dead simple.
Well, in a nutshell, an SuperGun usually consists of an RGB to S-Video, Component, or Composite video conversion PCB, a way to attenuate the mono audio off the Jamma edge to RCA stereo (which is not true stereo), power, test & reset switch, maybe coin buttons, and the ability to plug in custom joysticks for inputs. That should cover the basics of it.