I was thinking more like a normal friction drive wheel to provide rotation and cardboard cards. I think if we can get the rotation high enough and the launch speed fast enough, it should slice cloth and flesh and maybe even plastic quite efficiently. The cards themselves would have a lot of energy, but the weapon itself would be classed as a low-energy system. We could sell specialist metal edged or sharpened cardboard cards, but the idea of this thing is to use the ammo to hand. A pack from the petrol station should be good enough.
I do not imagine it to be armour piercing in any way, shape or form unles the cards are modified as you say, maybe a luxury silver guilded pack you'd find laying around on an 890j... With normal cards, if you do come across someone wearing armour running away would be the option B as Combat Armour would have a lot of weight. Even in Zero G a non-armoured person is going to be able to change their momentum a lot quicker than a battle armoured person would, either needing more energy to change momentum or more time = the card thrower still has a chance to get away unless the armour wearer has better thrust nossels, in which case damaging the walls of a ship you are in with your own weapon is probably not a big concern because whoever had boarded your ship has probably cut a big hole in the side of it anyway so in that case reach for the bigger guns.
Perhaps something with the friction wheel for the spin and initial launch, and a mini tractor beam arrangement to provide additional projectile speed and directional stability out of the launcher so no need for a bulky barrel? I would imagine this thing to be fairly small little bigger than a pack of cards itself so the unit in full would be nicely pocketable?
EDIT - This would be an option for the consumer market over total destruction wartime mayhem although i cannot disagree upgrades could make it in to something of a circular saw launcher...
