just try it and you will see that the copper percussion cap will be pierced with ease by the needle.this alone will open the way for the flame but be safe and fill the cap with fine black powder.
the action and the cartridge is not gastight but it handle this problem well. dont forget to have a good soft leather sealing gasket in the action.
chamber lenght is a problem for itself and I allways recommand to make a chamber cast. you will find that guns for thin paper cartridges have something like a conical chamber that going into the barrel without a step. stay with the 20 ga. magtech and you are fine.
use paper and felt wads in a diameter you get into your case without problems and fill it with BP and shot in equal shares by volume.

kuduae send me a link to an interesting double gun http://forum.guns.ru/forummessage/149/1846502.html

made after the patent of J.F.Timpe from 1884 probably by Emil Barthelmes, Zella-Mehlis for a russian nobelman. the gun have german proofmarks after 1893 and having "N.v.Dreyse Belin" on the barrels.

SN is 30925 what in a special way fits into the Dreyse Sömmerda serial numbers and do it also not.

Franz von Dreyse was dying in 1894 and the firm was coming to his son Nicolaus von Dreyse. it seems this happen between 34641 - the last gun I have with F.v.Dreyse and 34831 - the first gun with N.v. Dreyse. so the 30925 SN is much to early for the N.v.Dreyse name and I have my doubt that there was an own serial number block for the Dreyse shop in Berlin that only exist for some years in the 1890s.
its very unlikely N.v.Dreyse /Berlin sold in this time 30000 guns and giving them an own serial number. my own guess is the gun was honestly marked as "Dreyse Belin" because it was not build in Sömmerda but in Zella Mehlis ... but for some reason it got a SN that was open in Sömmerda because the original gun never came on the market in this time or the SN was not taken than.
we know that sometimes SN were reserved but this is all guesswork now.

another late 16 ga hammer gun that make no problems http://forum.guns.ru/forum_light_message/112/801988.html

N.v.Dreyse SN 35579

another late 12 ga double gun with an unknown action N.v.Dreyse SN 36620
had a bad time in WW 2 and after, stock looks like a crude homemade affair
https://reibert.info/threads/gladkostvol-n-v-dreyse-sommerda.452620/

here are pics of a Dreyse Drilling with the same action
http://www.littlegun.info/arme%20allemande/artisan%20l%20m%20n/a%20von%20dreyse%20gb.htm
to bad there are not more informations about it but Franz von Dreyse is maybe wrong. looks on the pics like three shotgun barrels.

another nice small bore stutzen, to bad without SN
https://www.dorotheum.com/en/auctions/cu...ch=Quick+Search