Use anything that will get all the bore butter out of the grooves. With a build-up, a complete seal around the ball cannot be made. Make sure that the thickness of the patch, compressed heavily enough to imprint into the pure lead ball, is enough for a full seal in the bottoms of the barrel's grooves. Square bottom grooves are easier to fill than round bottom grooves are. Always err on the side of too thick, as opposed to too thin.
Once past the muzzle, even a bore-sized ball will almost always be easily pushed down onto the powder charge. Assuming, the edges of the lands are not razor sharp, nor the bore rough for any reason.
Any lube with beeswax, or a synthetic wax, is subject to build-up in the bore of a muzzleloading rifle's grooves.
Bore Butter has an especially notorious reputation for build-up, being too stiff in cold weather, and for delivering less than the best accuracy at all temperatures.
There are simply better choices nowadays. For hunting, Track of the Wolf's Mink Oil Tallow is a very good choice. It will stay pliable on a patch down to temps that most of us won't hunt at (sub-zero) anymore.
For the range, I never found spit to be lacking, but a lot of people want a liquid that is more convenient. There are all kinds of concoctions out there, most seem to have windshield washing fluid, or window cleaner as a constituent. It's the ammonia in both fluids that people are looking for.
In case you are in need of more of it, this was at BlackSheep Sporting goods in Coeur d’Alene Idaho, it appeared to be a SMOKIN Good deal, You better jump on it before it’s all gone!
