The only thing I would add to the above two posts is, additionally, most designs incorporate a (flash fence - not sure of the correct term) that directs any powder/gas/bits of broken cap, upon firing, away from the face of the shooter.
On a drum/nipple system you can add a cup to accomplish this.
If you want the look of a drum and nipple, but also would like the functionality/safety/no 90 degree turns in the flash channel that you get with a snail you can go with "Ohio Patent Breech" from Pete Allan (sold at TOTW amongst other sellers).
It is totally cast/machined - so no threads to break off on the drum, plus you aren't drilling any holes in the side of a barrel which can be a safety issue, particularly in thin walled barrels (45 cal with 13/16" ATF comes to mind immediately).
I build with nothing but patent breeches - a safety thing on my part. Even a flinter gets a Hawken style flint breech and the flash hole gets drilled into the plug, not the wall of the barrel.
Anyhow, here's the Ohio patent (with a few notes that don't apply to this particular post).