As stated before I had to shorten the gun barrel to make it a L44 instead of an L55 Rheinmetall. So I did. I fixed the transparent tip of the barrel with epoxy as this doesn't put fumes on the transparent part. It was however painted afterwards so not really any reason to use epoxy.
Then I put some paint on so I could see the shapes. after two layers of paint I removed the molding edge of the tip and then I put some filler on the smoke remover and smoothed the filler.
After the filler, the barrel was painted three times and now it is ready to be put back on the tank.
Some pictures of it all: