Not sure if this requires a how to. It really is just an exercise in imagination.
But here are some of the details :
Body is 1" wood Handle is 2 pieces of 3/4" pine glued together with a 1/2" taken out of each side to fit the body into. The screws hold the body and handle together. I then use a high build primer and wet sand it to smooth it out and hide the wood grain.
I used a rasp and sandpaper to shape the handle until it felt good in my hand.
Barrel is copper pipe and fittings soldered together then a threaded rod run down the middle to mount it to the body. Could have used other material but wanted to have some weight to it so it felt more real.
The rest is just found object= co2 cartridge, wood dowel,pvc tube,some electrical switches,a spark plug,leather left over from bike seat build,apiece of a watch,misc. aluminum tubing etc. You just need to layer the parts together such as with the co2 cartridge it has some aluminum rings around it held on by brads glued into holes I drilled. Then each end is drilled and more tubes are glued in the front and a threaded rod with a twist nut is on the back.
I use 5 minute metal epoxy, brad nails, screws and contact cement to hold it all together.
Once you faux paint it it has a tendency to all blend together making it look like it one piece.
WARNING: Make sure any co2 cartridge you might use is EMPTY before drilling.
Hope this helps. Stuff like this is fun to do and a great way to help you think more creatively about your bike design.