The only problem I had with Magtech 12 bore shells was a reasonable amount of blowback into the action.
This was a bit of a pain as it meant removing the strikers (hammergun, so not so bad) on occasion to clean them.
However the blowback was gun dependant,my 16 bore had none so I presume they were a tighter fit in the chambers.
Circle fly is a good source of info and wads.
http://www.circlefly.com/html/bp_cartridges.html
I use duco to seal , as I could not find waterglass.
I suppose it would pay not to use any glue that might stick to the brass too well.

GDU