12 ga rolling block

Does anyone here know if the later modle rolling block shotguns have thestrength and overall size to be chambered in 12 ga I have been told the 12 ga are not really 12 ga and I wanted to build a 12 ga 3 inch slug gun on this it has been a on again off again thing but never got off the ground. Dont want to go to all the trouble to rebarrel if its going to be unsuitable. there is one gunsmith I have talked to said do it and another said a 20 ia all that can be done mine is supposed to be already in 16 ga but no markings to prove that any help will be appreciated

Medicine Hat