Now here is my idea for a really easy big bore. just use a .410 SAGIA as is but with a rifled barrel. use brass 2.5" shells or possably those steel gold bear and silver bear cases and load it with protuding slugs/bullets into a 3" sagia Mag and you would have a pretty wicked big bore for the price of a barrel and could be left with a shot gun designation and would still shoot 3" normal shotgun shells.
hell in some states you could even hunt with it in a shot gun zone. not mine they dont allow 410,s on dear in wisconsin.
Sagia 410,s are realitivly cheap compared to every thing else right now. High capacity mags are avalible.
barrel blanks are avaliable pretty cheap around $80 for a 15" blank add a much recomended break and you got your 16" for a rifle build ( hmm this might still be a shoot gun so the barrel might need to be longer)
.41 Caliber Cartridge Pistol Blank
I am thinking this might not have the feed issues of the 444 marlin that tromix built and it would still shoot normal shot gun shells so you could use it multipul ways.
also brass 410 shells can be made from 444 marlin, 303 british, and 9.3x74 russian brass..
was also thinking this would be a good round for shooting cast lead bullets.
the 410 Sagia will handel the higher pressures so a souped up 410 rifle round might really be a easy way to go here.
Kind of a Mini SGFH type deal. any way just thinking out loud. this round would be very simular to the old 405 Winchester
or the .40-60 maynard