tinker I can only believe that your gun is uncommon large. I have never had any problems with magtech brass, cant find that it "weak" or soft. I load 90 grain ffg in a 40mm long magtech brass case under a 480grain minie bullet. the bullet is resized from .585 to .605 in a home made hammer swage die to fit bore diameter.

my chamber is so tight that I cant load a cut down magtech case. it needs a resizing work in another die to press it down on the case mouth from 15,60mm to 15,47mm. this is not so much but the brass won't load without.

28ga magtech brass, 50mm long with bullet and loaded cartridge, 40mm long case-loaded
the 50mm brass is the original swedish version. I have try to load a long bullet in this case but the chamber is made for a roundball. I load the bullet in 40mm "german" brass with a .577 snider die.

I have loaded some cases the 5.time now and it dont need another resize work. the brass is annealed and I dont believe that this is a wimp load!

have a .24ga hammergun that accept 24. fiocchi plastik but is much to tight for the 24 magtech brass. I believe that the brass is 0,3mm to large on the base to fit the chamber. I turn this material from the base,anneal and resize the case in another home made die. than press it into a .577 snider die and I be able to fireform the brass.

you fauneta pic is very interesting, I was believe before that the fanneta and explora magnum bore cartridge had have a very strong brass case.It looks very, very thin.
seem's me that your inner paper case is a lot of work. will it stand against the black powder heat? I like to see some pics from your pinfire cartridges. have make such things some years ago.
have shot 40mm cases in a 16 bore made from 18mm copper tupe in my wild days. that was when magtech brass was hard to obtain here.
you can imagine what happen with such a loose fit.