Skeettx, of course those Briley tubes are an excelent solution, but I can not import them. Besides, they cost twice what I paid for the Darne (probably the best deal I make buying a firearm!)
Stan, I think that the POI will not change significantly. The inserts will be about 10" long, I will make an "inverted forcing cone" at the end to smooth the transition of the charge between the insert and the original barrels.
As I understand, the plastic wad will seal gases once it is on the 12 ga barrel and the original chokes will control the shot spread.
Best,
