How about an existing slug of 375 gr.(Lyman 20 ga) launched
out of a 12 ga. platform ? It can and has been done. I'll give
you a few starting clues.
Lyman 20 ga.Sabot slug (Get the oversize ones from Gardners)
Resin filled cavity with release agent in void -Coat of wax
will help the plug pop out when fired. Knurl before filling
Knurled rear driving band only.
Winchester Primers (209)
Powder (now this I am still playing with) WSF or Blue Dot.
Federal Gold Medal Hull (Fold Crimp)
Federal Wad (Punch a 1/4" dia. hole at bottom of wad slit)
(Use the thick walled brown ones.) 12S4 Mica dusted.
The Duplex Sabot from the wad cup of a BPI LBC wad.
Yes .you will have to trim it to size to wrap around. Thick end
towards back. They are tapered in thickness. Split in half
About 40lbs wad pressure.
Are these accurate ? Well yes and no. Short Range has
not been a problem (50 yd) . Beyond 100 yds it is a crap shoot.
The real problem is the rate of twist/velocity of the 12 ga barrel. It is a bit too slow at 1-35 for the weight of the slug. The quicker the twist the better the long range accuracy. It does improve in a 1-32 so I am sure the benefits are there. I am curious if anyone has tried this combo in a 1-28 twist barrel. I do not have one in that configuration.
Let me know. I have been at this for a couple of years.
I have NOT found the ideal combo YET !