If you are dealing with paint on plastic something you may want to try is SuperClean. It's popular for stripping paint off wargaming miniatures as it doesn't eat the plastic and it works very well. As for the paint on the outer barrels I'm not too sure if it will work or not.
You can get it at most Canadian Tires. Just let the painted part sit for a few hours and then scrub with a tooth brush.
Edit: Another product you can try is Chameleon model paint stripper. SuperClean seems to work just as well though and is way easier and cheaper to get.