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12 gauge card under the 12 gauge wad (between powder and plastic wad).

Putting a Gualandi Over-Powder Wad (actually a card disc)
http://www.gualandi.it/en/products_discs_pads_various_items/white_over_powder_pads.html
under the Bior-Pallettoni 20mm wad...

What is the diameter of the slug at its widest point?

Randy,
If you try them in a smoothbore cylinder choked barrel I think you will be pleasantly surprised.

https://www.ballisticproducts.com/12ga-DGS-12-Slug-25_pak/productinfo/0721612/
https://www.ballisticproducts.com/12ga-DGS-Thunderbolt-slug-10_pak/productinfo/0721613/

The only such slug I know is the Sauvestre but it uses an undersized slug and a sabot that also works as a gas seal.
http://www.sauvestre.com/12-70,301?lang=fr

Some simple but effective crimp bases:
http://www.siarm.com/index.php?cPath=2_105_103_156
Some manual roll crimping tools:
http://www.siarm.com/index.php?cPath=2_105_19

For a crimp like this:
http://www.gameandfishmag.com/editorial/are-you-using-the-right-slug-for-your-shotgun/245179#
You have to get a crimper like this:...

I think that a small hole at the center of the "shotcup" will help to put the slugs in by thumb pressure only.

You can use these wads, unslit, for even tighter patterns or slightly cut (about half length, 4 petals) for closer range:...

http://www.gualandi.it/en/tables.html

There is another option. Benelli Super Nova. Chamber is 3", not 2-3/4".
https://www.benelliusa.com/slug-guns

http://www.chedditeitaly.com/en/inneschi.php

A 9.3x74R shell holder also works with Cheddite hulls.

http://www.gualandi.it/en/tables.html

Didn't have any accuracy issues when loading the Lyman sabot slug or any other slug in 2-3/4" hulls and using it in a Benell Nova with 3.5" chamber and smoothbore barrel.

Maybe a Savage 212 with a heavy barrel, chambered and bored at minimum (pressure barrel) dimensions would qualify.

What I found with the RSI Pressure Trace is that replacing the Loctite 401 provided with Bison Epoxy Universal (two-part) strain gages are much better attached on barrels.

http://svaroghunt.ru/pulelejki

The only problem I found with Gualandi slug wads is that when loaded at maximum velocities (about 2000f/s) the wad's base turns inside out, due to muzzle blast I suppose.
Solved the broblem cutting...

Almost all Brenneke slugs are available as components. In Europe at least.
http://www.siarm.com/index.php?cPath=2_20_164_60

Beware when loading 2 or 3 ball loads. Balls must be absolutely in line each other (not staggered).
I' ve seen bulged barrels (at forcing cone area) with balls not put in line.

I do not think that Gualandi will ever sell their slug wads as components only.
If I had that mold I would make a new base pin and make the following wads to fit, with petals removed....

If I had the gun I would first measure the bore to confirm the .775" diameter.

Try it with strong primers (CX2000, 616, 688) and you will get 300-400f/s more velocity with the Lyman sabot slug. I use it exclusively in the 12ga for every load from 32grams and up.