i never had bore butter build up in bore,BUT i clean it with warm water after each shooting.if you use HOT WATER, you will get rust.then put bore butter on that rust, oh my.soooooooooo,if you use hot water as rb and cayugug said, you have to dry bore quick. i rinsed my bore with hot water, ran down cellar steps to my bench and it rusted that quick.rb called it FLASH RUST. i never had that with WARM WATER and i still got the old bore butter out of bore with ,no rust.i think warm water will get bore butter out IF you clean it when you get home. let it sit for days, hot water i feel is needed.only my opinion from using warm water and no rust for 35 years.