Aaron you could shorten Rocky Mtn brass cases and crimp in a groove,
but plastic cases are only pennies each while good brass ones are
6 bucks each. Seated and roll crimped in plastic it will still have
the same room under the slug for powder as a real short brass one.
And the pressure our slower powder loads has is ok for plastic.
And cheap brass cases like the baloon head Magtech or others
aren't any stronger than plastic, For sizing my brass cases long or
short I use some big Hollywood 12ga dies that are threaded for
my big ammomaster press. Do you use a roll crimper that fits
in a drillpress. Tha's best for plastic.Ed