those black stuff is orginal lubricant

I have once ( may be twice ) fired it without closing the breech and lubricant was ejected through the seal between the breech and the cyclinder wall like hot spring.

I think the old ring will need be replaced in the future. Is there any special requirements for air guns ? I have only come across two types of O-ring. One kind is VERY cheap and they are always black in color. When put under pressure for a long time, they will gradually deform and cause leakage. I can get this type in almost any sizes from local shops. The other kind I know is made of silicon, they are softer, more expensive but last a lot longer and can withstand higher temperature. They come in all kinds of color.