Having now looked at both elements required to start the design process I've been thinking about how the parts of the revolver gun can be incorporated into the bike's frame and parts. The main parts of the revolver that I'd like to use are the gun barrel, bullet cylinder, trigger and hammer... maybe I'll try and throw the grip (handle) in there somewhere too.
I started by finding a good reference image of a revolver gun and using Photoshop to cut it up into pieces and just sticking the parts on to a frame of a motorbike. This helped me to decide what parts would be effective in the design and it was an easy way of moving the parts around and adjusting them.
Here's an example of the basic cut and paste technique: