Hello!
When you are scratchbuilding, you can do ANYTHING! Question is, of course, what will it take - depending on how detailed you want to be. On small scale models - 1:72 or smaller most of the time you don't need every small detail - you just have to make the impression on the viewer, that it is there through right painting and such. On bigger models you want to do more and more actual details. First thing is to research - you need to know what the real thing looks like, even if you don't reproduce every wire, bolt and rivet in the end. Then you do the building - it pays off to look for aftermarket stuff first. Life is short and if you can buy at least a part of what you need instead of spending time to build it, it's worth thinking about it - like if you could buy Browning machine guns in th right scale, you'd already have important part of your wing gun bays there, wouldn't you? You also need to start looking for materials - like styrene sheet of different gauges, soldering and copper wire, and whatever might come handy - most of the time it's stuff you'd normally throw away, like ice cream boxes (if they are styrene it's free sheet), scrap electronics (for wire but not only) and so on. In the end it all comes to getting some basic dimensions (opening in the wing, size of the machine gun - things like that) and cutting and glueing tiny pieces of styrene and wire to match your research material in the end. If you want, you can take a look here:
http://cs.finescale.com/fsm/modeling_subjects/f/16/t/173369.aspx
or here:
http://cs.finescale.com/fsm/modeling_subjects/f/3/t/165990.aspx
where I go bananas with such stuff.
Building a paper model before also helps a lot - develops your skills sharply.
Good luck with your builds and have a nice day
Paweł