why do you have to mix tin with the lead... why cant you just make the bullets out of pure lead. I know they would be softer... whats the downside of that just shooting paper..
If you use "pure" lead for bullets, except for muzzle loaders, you will be limited to very light loads that produce very low velocity. If you try to push them faster, they will deform when fired and produce leading in your barrel. There are some .357 diameter, hollow base wad-cutters (that are swagged, not cast), available from various manufacturers that demonstrate pure lead can be used albeit, with low velocity target loads.
Mike TX gives a very good explanation above concerning the benefits of alloying Lead for bullets.