Finding real slow powders that works with shotgun primers,
without starter powder. We found RE-17 and surplus 7383.
To recap faster rifle powders we use -VV110,IMR 4759,4227..

On the thread we've written about real slow powders like
H-BMG we've used with a starter powder, IE 15gr of Blue dot,
and H-BMG in amounts from 150 gr to 230gr in 3.5 and 3.85
inch brass cases. And 3.5 " plastic cases. 860-1040 gr slugs.
Tested in NEF with long heavy barrel.

We finally got some RE-50,a really slow powder like H-BMG.
It works with a real magnum shotgun primer with heavy slugs,
860 gr on up, in 3.5" RMC brass, 3.5" plastic and our long
brass case. I used 239 Fed primers. Lighter slugs
only give squib loads.RE-50 Is a powder made like RE-17
with the deterrant all through the grain of the powder,
rather than all on the surface of the grains, so it will ignite
easier, to get the initial combustion started.

In brass case used 240gr with 860gr slug, 1950 fps
and 220gr with 1040gr slug, 1750.
In 3.5" plastic 860gr slug with 180gr RE-50, 1500 plus.
Now these velocities aren't earth shattering as powder is real
slow, but cases don't expand, not even plastic case base cup.
And no starter powder. Nice thing is just fill cases
leaving room for slug and seal and no worries about
to high of peak pressures with the heavy slugs or
cases expanding to stick.

Now to get 239 primers get Fed 12ga long red
3.5" primed cases from BPI. I used Fed cases,and took out
primers and used in other plastic and the brass cases.
Got a big shop trying to free up 239 primers for us reloaders.
Federal puts them in 3.5" mag 12ga loads and 10ga cases.

Elsewhere it was reported someone may do a 16ga sabot.
Also a guy got huge antlered buck with hotrod 20ga. Ed