hi allyne , i would agree that the german cannons on the fw190 look straight not tapered, i usually replace any gun barrels i can easily get to with brass microtubing. Ive got a selection of sizes so i just tend to pick the one which looks about the same size as the plastic one im replacing ( in other words i just guess it and hope for the best!!) A great place to get the tube is an e-bay shop called folly models , i think they stock the albion alloys range of tubing , you should get what you need from them . As for the original question , i would get a few lengths (they come in foot lengths btw) of the 1.2mm and a few 1.5mm dia brass tubing and see how you go. hope this is of some use cheers tony