I am guessing with the card set up it would be similar with a 20 gauge slug except it is a round ball.
You are right that it will be "similar" to a 20-ga/. slug, but due to being solid instead of holllow, will penetrate a lot better that the 20-ga. slug, and do a lot more damage, too! I would take your load any day over any 20-ga. slug I am aware of!