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 will take the time to look and read through the links I posted above, you will find plenty on what tin does in the alloy. But for now here is a very cut and dried explanation.
Tin in the lead will for the most beneficial part allow the lead to flow more freely and fill out your mold better, and give the lead a touch of hardness and malleability. Pure lead, or lead with some antimony in it, (Wheel Weights) will work just fine for loads running in the 700'ish fps range up to around 1200fps. However this will also depend on the condition of your barrel, the fit of the bullet, the lube your using, and the powders your lighting off under them. The softer the lead the slower you usually have to go.
When folks recommend adding tin, it is usually for better fill out as mentioned above. That said it doesn't have to be pure tin, it can simply be several inches of 50/50 up to 95/5 solder. The trick is to add a little bit at a time to your pot, noting how much tin your adding to how many pounds of your alloy. Once you have it mixed in really well and fluxed, you pour a few bullets and check how they have filled out the small corners and ridges in your mold. Once everything is sharp and detailed that is all you need to add.
Tin will also help bring a high antimony content into a much less brittle and more malleable alloy. This will give you a stronger bullet which upon impact instead of shattering or fracturing, will flow back resulting in more of a controlled expansion. For instance, the Isotope core alloy is 1/3/96, (tin, antimony, lead) While in this form it pours wonderful bullets, but they are a bit brittle with regards to expansion. With the RFN designs it works great, with the SWC designs you start to get a few shedding their noses, with a HP you get the nose blown off and just the remaining base is about all that is left.
Add in enough tin, and a little more pure lead, to bring the percentage up to equal the antimony, (in this case I brought it up to a 1.5/1.5/97,) and it will change the whole deal. Now the RFN's will roll back a bit giving some show of expansion mainly around the very end of the noses, the SWC's will now more or less roll back to give a nice flattened nose back to the shoulder of the first driving band, the HP's will now roll back and hold together into the fine mushroomed shape they are supposed to be in.
(similar to these,)
The links above go into much more detail on what to use for what, and how to blend in whatever is needed to get the overall desired results with your alloy.