naa, your guys have to leave your little word and get a bigger picture.
you probably know that chuck norris once made a mistake? it was when he thought he was wrong!

well, I was wrong believing the article description when knowing they are wrong most of the time when it comes to antique guns. without having the time and the space to show the teschner cartridges evolution from 1859 onwards simply saying:

this is a Teschner-Collath Cal. No. 3 rifled gun

it having nothing to do with 600 NE or such crap because it originated in 1859

here is the only No. 3 shell I have at home

the green paper shell

not the nicest but you get the picture

Teschner Collath shotgun shells have nothing to do with Lancaster caliber but you find a basic case.
the green paper shell is the No.3 , then comes 16 ga Magtech, 0,70" v. Dreyse and a 20 ga magtech resized to fit the Dreyse chamber.

you see here a home made die, very simply just to reduze the 20 ga diameter so it fits into the dreyse chamber.

what would I do? 16 ga is the closest
No. 3 rim 20,75 mm
base 19,48mm

body 18,10 - 18,20mm

you see over 15 mm the diamater goes at the base down from 19,48 to 18,20mm

16ga Magtech rim 20,44mm
base 18,76mm
mouth 18,45 mm

with only the rim and base in the water aneal anything of the magtech brass. made a die with 18,15 mm inside. you see mine its only 20 mm long inside to reduce friction.
dont use a shell holder, it will not work. I have a simple plain shell holder for such work where the case stands on. lube the anealed brass and pressed carefully into the die. you have to find out how much is neccessary to get it into the chamber. it will be looking similar to my 0,70" /20 ga.
remove the brass with a fitting steel tube and a hammer out of the die.
its a blackpowder gun, I would not use other than this. with blackpowder pressure I think the base a little bit undersize is ok. show a chamber cast, could be the brass is to cut down.