If you know the "smith" who rebarreled your mauser, I would confront him. I don't think you have any choice but to have that barrel removed.
I fear that whoever put that barrel on your action didn't know what they were doing. I am relatively certain that the threads on that Win. barrel do not match the FN receiver, hence the need to fit a setscrew into the receiver ring. Sounds pretty fishy to me! :eek: I could certainly be wrong about this, and for your sake, I hope that I am.
I'm sure that I would not have the same person try to correct the problem. Find a qualified gunsmith! This could be a safety issue!