I tested TC bore butter on a .45 custom flintlock for over a year. I fired the rifle one or two times a month for a year and never cleaned. Would shoot then reload with a bore butter patch. After loading I would run one more patch with bore butter down until touching the ball and then back out. At the end of the test time, I had not experienced any problems with rust. Sorry that I traded the rifle off. Still use bore butter but now I do clean rifles after a shooting session. During a hunt, I continue to use just bore butter without cleaning until the hunt is over.