You could use the Remington 8ga plastic kiln gun cases,
like I used in my 8GA FH bolt action Enfield --
Check the 12 GA FH thread in big bore forum. Cost is only
35 bucks for 50 all primed. Ballistic Products.
They are double layered base cup for super pressures
and just turn base down a liitle. Hunters in UK do this and
this case is so much stronger than soldering tubing together.
The heat will weaken brass and copper. My top load is
1050 gr slug at 2000 fps and the base cup expands
only .001". ed