I've run 110 grs. of FFFg under a 0.690" RB and that worked well (not accurate but big boom and good combustion!) but of course more than somewhat lighter than your slug.
Take a look for turbo1889 with the search function. He posted a lot of info on loads and used Steel a lot. There may be something useful there though not sure he went as heavy as 770 grs.
The Blue Dot load I used under the 800+ gr. slugs was based on an Alliant heavy shot load. I haven't had a chance to look yet and am not sure I will find it anyway but it was an equal weight of slug for shot load.
Looking at my old Alliant reloading book, I see 1 7/8 oz. shot over 34.5 grs. Blue Dot at 10,700 PSI. Now I was thinking I used 33 Grs. of Blue Dot under those 800+ gr. slugs so may have used the 3" data for 2 3/4" hull. I didn't blow up and it worked pretty well but best to use 3" hulls for 3" data. I know I used 2 3/4" hulls but not positive about powder charge.
Someone else you might do a search on is Ed Hubel. This thread may have some info:
http://castboolits.gunloads.com/show...FH-Slug-Tester
There are 35 pages to go through!
Ed has posted load info in other places as well and is an advocate of very large charges of IMR4227 for high velocity slug loads at low pressures. He has worked up some pretty spectacular loads to say the least. Best to do a search or PM him. I am sure he'd give you some advice.
Longbow