I re-read the posts and am a bit concerned in regards the twists and shallow rifling. You may be stuck with slugs - oh well. I know you'll make the most of it. If shooting bare slugs that run groove or slightly larger, I'd apply some bullet lube to their sides. Nothing leads worse than a bare shotgun slug in a dry and clean or foued bore.

I'd even try MS Moly on the slugs - I use it for my gopher gun's bullets, a .17AH and they work as well as factory moly coated bullets.

Otherwise, I'd use something like SPG or Lyman's BP Gold for smokeless or Black, or perhaps one of the 50/50 alox/beeswax normal smokeless lubes. I really like the Lyman's BP Gold. It even works well for full power loads in my .44 Wheelgun and .50 Alaskan RBlock - no leading, even up to 2,000fps in the rifle.