Re: Black Powder Pistol ?
Kroil works well to free up rusted parts another good one is Wurth Rost Off it comes in aerosol cans. There is a new version called Wurth Rost off Max Ice here's the sales pitch. We use it at the dealership to release rusted on exhaust studs etc. I don't think it will damage the finish, but to be safe dab a little on a spot that is not visable. Kroil is safe on gun finishes. I find it best to let it sit or soak for a couple days.
"This product represents the latest breakthrough in penetrating lubricant technology. By combining rapid freezing action with an advanced, rust eating formula, seized or rusted parts are easily released. Shrinkage of sprayed parts quickly breaks open micro fine fissures allowing the lubricant to easily penetrate through the toughest rust. The innovative rust eating technology then dissolves rust making the job a snap and future maintenance easier as well. "