Tim, your problem is an interesting one. When thinking about new 12 ga. barrels on the existing standing breech, it seems to me that the new barrels would have the same dimension problem as the existing barrels do if the new barrels were made to fit the standing breech as it exists. That is too little thickness between the two barrels at the breech. Is it possible that the only choice might be a smaller gauge whether the gun has new barrels or sleeves ? Sleeves might put a small bit of extra weight "between the hands", but of course , inch for inch with similar wall thicknesses, 16 ga. barrels are lighter than 12 gauge barrels.
