I've used 4759, H4227, 7625, 4756 and BlueDot, for BRI500, Lyman 525 sabots and 600-1050 gr slugs. (7383 and RL17 are being used for some of the heavier slugs in plastic and brass cases...that is covered on Accurate Arms)
H4227, 4759 are the slower powders, require more powder, have better fill characteristics(no or less filler wads) have higher velocity at lower pressures. BlueDot is faster, worked a bit better in 2 3/4" cases, no need for fillers, and in rolled crimped 3'" cases at higher powder amounts, but produced too much pressure for the Mossy and 870.
BlueDot is also infamous (notorious) for causing much grief in certain applications up near the top of it's pressure range...at least that's what I've read and I've had a few pressure "excursions" with it in plastic cases and heavy slugs. H4227 and 4759 are much more forgiving powders, produce all the velocity I want at lower pressures, so I use those two and RL17 for most of my 12GaFH load work for ALL slug weights.
Follow BRI's data with their slugs and you won't have any problems.
I developed most of the loads in Rem 870 and Mossy 535 using 3" and 3.5" plastic cases so the pressures are equal or lower than factory...basically equal to Dixie Slugs Terminators...all the information is/was on Dixie Slugs web site.
In my 12 USH rechambered to 3.85", the 12GaFH using 50BMG brass, things are considerably different. If you're interested in visit Accurate Arms, Big Bore, 12 GaFH thread. Read the many pages and you will get an idea of the potential of the USH.
I won't post any of my data, but I worked up some very nice loads that were more than adequate...many approaching the same weight bullet velocity in the big bore rifles. Much of the reloading data came from BRI's reloading pages and from Lymans shotshell reloading manual...some came from Load from a Disk...remember the shotshell is just a case with a certain volume and the slug is equal in weight to ~1 1/8 oz of shot...You can use data and powders from shotgun reloading manuals for that weight of shot starting low and working up slow...some data came from Ed Hubel and others on Accurate Arms Big Bore thread and some from load testing.
The LYman 525 slugs you can get from Gardners Cashe, expanded to 0.860" and penetrated over 30" in Lodgepole pine rounds and the BRI slug didn't expand much but held its shape and kept the plastic "tail" on and penetrated as much if not more.
I DON'T ADVOCATE LOADING BEYOND SAAMI SPECS IN ANY RIFLE, PISTOL OR SHOTGUN...TO BE SAFE.
You really DON'T need to load these two slugs to recoil punishing levels to have very effective results, but you can load to the NEF receiver's potential and safely exceed factory 12 ga slug ballistics by a bit, and in the rifled USH, accuracy is plenty good for 100-125 yd shots.
It's your choice, but Be SAFE.
Luck
