I have a newport w break open 410. The only markings on it say 410/12mm choke. I can not find a chamber size anywhere on the barrel. Ive shot hundreds of rounds thru it over the past couple years and may have already shot 3" shells i cant remember but im leary to do so without knowing the chamber size. Without knowing if the choke is full or modified im leary of any slugs. Any help would be great.