After the screw in the barrel is removed, the barrel has residual temperature. When the residue of the material is soft, use a cloth roll to clean the barrel. Illuminate with light and check for abnormal scratches and scratches on the inner wall of the barrel. The change of the inner diameter of the barrel is measured, and the wear condition is judged when compared with the inner diameter of the barrel before use and before the use. Then check the rest of the barrel for damage. 
Do not apply oil and rust after cleaning the barrel.