Smith carbine cartridge question.

[Mr. 16 gauge]

I was at the rifle range on Sunday when a couple of reenactors (Civil war) came in with a replica Smith carbine. The gun was interesting and the shot it in preparation for an upcoming competition.
The cartridges the guy was using were plastic: .50 caliber conical on one end and a flash hole on the other covered by paper and 30 grains of BP in between.
I asked him if they used plastic back in the Civil war and he said no. I asked him if it took a brass case or a paper case, and he said "Neither.....it was brass and rubber

I never heard of such a thing.....I've tried to find some pictures on the internet, but couldn't find any. Does anyone have some photos of this interesting concept, and what purpose did the rubber play?