6mm x 1 is the metric nipple. They can be found at RMC Sports. #ML094 http://rmcsports.com/catalog.htm

The rifle actually does have a breech plug, but it does not come out. It is not removable is what I mean. There are a number of different ways to clean a traditional rifle. Some use a water bath, others just patch them clean.

To check the bore you need a bore light. Another way is to take a piece of tin foil and roll that in to a ball, SMALLER THEN THE DIAMETER OF THE BORE. Drop that down the bore then shine a good flashlight to the side down the bore and it will reflect off the foil. Its not the best way to check, but it does work.