The German caps are "RWS" and prior to the introduction of CCI's "Magnum" caps they were the hottest cap available.
IMO, the "Magnum" caps are about the same and I recommend either if your shooting one of the new replica powders like Pyrodex or 777.
Speaking of those replica powders, they all have a higher ignition temperature than black powder so if you end up shooting one of them, be prepared for an occasional "miss-fire" or "slow fire".
Assuming your buying a percussion style gun you might want to remember the following:
If one of these "miss-fires" occures, wait at least 10-15 seconds before you begin to think it's not going to fire.
I've never had one take that long, but I have had a delay of up to 3 seconds before the gun actually fired.
I agree with the answer about the snail.
There are two common styles of attaching the nipple (where the cap goes) to the barrel.
The "drum" which is a cylinder sticking out of the side of the barrel is fairly common with original guns. The nipple screws directly into it, and it (the drum) is supported by the lockplate when the barrel is installed.
The breech of the barrel is sealed with a simple screw plug which has a integral tang for attaching the barrel to the stock.
The "snail" type breech plug is the other common style. With this style, the nipple screws directly into the breech plug and the flame from the cap travels thru a hole that goes down to the barrels bore.
Many breech plugs of this design are called "hooked breech plugs" because they have a hook at the rear which engages a seperate steel block with tang. This design allows the owner to remove the barrel by simply removing the barrel pins or wedges and rotating the barrel upward to disengage the hook.
Some of the breeches do have a snail design adjacent to the nipple and some just have a flat surface at this location without the snail.
Almost all of them do have something to direct the flame from the cap and powder discharge downward and away from the shooters face.
There is much debate as to which style is "best" and both have some advantages over the other.
I don't think this is the place to debate that issue and I hope this answered your question.
zonie