I have 2 of the H&R 12 Ga slug guns. I have tried many brands. I get best results with Hastings slugs.
see: www.slugsrus.com
I have spoke to the design engineer himself, they are a GREAT round.

They are a cheaper priced slug and if you buy them at: www.grafs.com
they are the most economical there, because they have a fixed one price shipping.