Search for terms such as 'Broadwell Ring', or 'Broadwell Breech Loader' and actual weapons will show up, which often makes things easier to understand.
I had to correct the former's Wikipedia entry as it said the L30 and M777 guns used "sliding wedge breechblocks with obturating rings", which is false on both counts. The former is a vertical split-block with an obturating pad, whilst the latter is swing-up interrupted screwthread design with an obturating pad. I have found that technical weapon stuff on Wikipedia is often incorrect, or written (it seems) by people with a poor level of technical knowledge.
As to what ammo the guns would use? From the look of it they (there are four-, five- and six-pounder guns) would all use separate loading ammunition. So a projectile, a bagged charge (black powder), and by the look of it a tube-type firing system. The tube being a small but long tubular primer, which would have a friction igniter system at the top, whilst the rest of the body would be filled with a fine black powder igniter charge.
Yanking a cord attached to the friction igniter system sets that off, which ignites its igniter charge. The igniter charge's output vents into the chamber holding the main charge, igniting that. The pressure builds up to a point at which the projectile starts to move (the shot start pressure). The projectile's movement due to its diving band (s), base cup, or both being deformed due to the force apply to the its base (pressure x area = force), then the overcoming of the static and dynamic projectile-to-bore (lands and grooves) friction.