I have one of those 12g barrel inserts in 45LC/410 and I'm collecting parts to build a No1 Mk3 Enfield carbine for pigs in 45LC using an M1 carbine mag and some assorted bits of stocks I have kicking around.
Haven't had a chance to try the insert yet but I have just bought some Starline brass and 250gr hardcast Blackwidow pills.....time will tell

Roger