Actually they surmised that the silver bullets were unaffected by the devils power because they were hallowed thereby proving that rifled barrels were more accurate due to unholy powers at work. It was then declared that rifled barrels were hexed and anyone using, making them, etc.. would be charged with some sort of heresy or witchcraft and put to death. The truth is that the lead being softer filled the rifling grooves when being fired in a black powder rifle while the silver being harder did not fully fill the grooves. It is said that the brave hunters in large part used rifled barrels anyway. Possibly it had something to do with the choice between maybe getting caught by "holy" people or starving to death due to missed shots. It's refreshing to know that even in those dark ages people were not stupid enough to believe that nonsense. My opinion is in a modern smokeless firearm silver bullets would probably work with the right powder charge, weight, etc... workup but not any better than other MUCH less expensive alloys of similar weight and hardness. In a black powder firearm I would guess they are too hard. The Yankee Marshal has some youtube videos that include a box of silver bullets and information on who sells them.