The barrel's breech plug could be threaded into the back end of the barrel, or simply slid in and head-spaces against the rifle's rolling block, just as a ctg. case would in a CF ctg.

The TC shotshell primer seat is threaded into the centre of the barrel's breech plug. The firing pin hits the inserted primer and POW!

The breech is still 'taking' the pressure just as if there was a ctg. in it - only this time, the ctg. is the threaded (or not) breech block in what would be a 'chamber' in a CF rifle.

This is a very simple system.

In the past, some guys, when wanting more powder capacity than the BP ctg. could contain, have used a chambered empty ctg., action at 1/2 mast (cock), like a .45/70 brass case, then poured in a measured powder charge- 80 to 120gr.of powder, then muzzle seated a bullet on the powder - then fired. This was done by Paul Mathews friend and gun smith using a rolling block rifle.

This type of loading is usually felt to be unsafe (pouring powder than loading a ball o bullet into an already cocked rifle, 1/2 bent or not and is generally not recommended by today's slightly more safety conscious crowd.