Identification please

I found this in my ctges room.
Because it is a mess and furthermore I’m loosing memory, I don’t remember what it is !!

It is empty and has a hole at the rear

It is in copper (diameter 7.80 mm) and the black back part (diameter 7.90 mm, which is crimped on it, is in steel

Total length 44 mm
copper length about 36 mm
steel length about 8 mm