Wolfgang,
If the barrels don't shut, I can't see how that would be caused by shooting the BBF barrels.Without seeing the gun,I can't identify the cause of the problem, if there is a problem. A new set of barrels would normally be fitup pretty tight, and since these haven't been shot much,they might just be "stiffer" than the others.Since he is going to have the gun renovated,I suggest he talk to his gunsmith about it.If there is a problem,it would be much easier to repair, than if it was too loose.
Mike