Bore MopBore Techs Proof-Positive Bore Mops are densely packed, 100 % cotton mops that can be used to apply cleaners and oils to the bore or for polishing and drying the bore. All mops feature non brass cores and threads to prevent any false indication of copper fouling during use with copper removing chemicals. Description:.40 - .45 CaliberThreads; 8/32Length: 2.6