When soldering copper pipes together, the outer piece has a position that the inner piece stops at. The washer prohibits one from pushing the brush in too far allowing for only the soldered area to be cleaned.
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Life Member NRA
Life Member NAHC
Former President of the ECPT (Eifel Combat Pistol Team)