I discovered that watch spring bars/pins used for wrist watch straps make excellent gun barrels. You can get a set of them in different lengths online for about $6, with free shipping on eBay. (See photo).
First, choose the length you desire. Don't try to cut the barrel to length, as I discovered you will only succeed in crushing the bar/pin. To remove the pins on either side of the bar/tube, you will need two pliers to grab the tips on both ends. Pull, until one or both come out. The remaining tip should fall out. If not, clip it off, but not the tube, as you do not want to crimp it.
A tiny spring inside will slide out. Keep it for your bits box. You now have a perfect open-mouth gun barrel that is close (Well, closer anyway) to scale. Would work great on a needle gun. Notice the car with the
MG on the side has a barrel attached to a Orlock auto-gun, and that it has a opening on the end of the barrel. Neat, no?
EDIT: Craft tube beads are also great for larger bore guns. Ignore that some in this photo have crushed or rough ends---They usually come in perfect round shape.