My Dad had a Hastings Barrel on his 1100 12 gauge Remington.
We always used 385 grain BRI sabot slugs before they got bought out by Federal or Winchester (can't remeber which). That combo was deadly. He shot a doe at 142 yards with it and she dropped in her tracks.
I got the information from Rich ( http://www.slugsrus.com/ ) who was the org mfg and designer for the Hasting slug. He did mention that he might not have any of the RWS loaded ammo until Nov.
He does have the loose slugs and equipment if you want to load your own.