I dont think its a good idea to have a card before a slug when it goes into the barrel.
sometimes, something is maybe coming together on the bad side of life.
I melt paraffin and fix slugs with this and a small paint brush. believe paraffin is also soft enough when goes into in the choke bore.
you can crimp magtech brass but I recommend to anneal the case mouth because its to hard and will split most times with the first shot.
have the old RCBS 12 ga reloading die set they dont make anymore and it works fine