Pewter can be used exactly the same was as tin. (not roofing tin, or tin cans, there is a big difference. Those are actually coated steel).
Tin is expensive. Normally, bullet casters get tin from solder, which is expensive. Tin sells for about $8-$12/lb. Normally, making a bullet with any sharp ridges or several small lube grooves is much easier using a little tin in the lead mix. It breaks the surface tension of the lead and allows it to flow into the grooves better. (About 1.5-3% in the mix will do ya). So if you can get a couple of old pewter mugs, pitchers or picture frames at the thrift store for a buck or two, and end up with a pound of tin, you come out way ahead because then you don't have to buy solder!
Round balls or a round nose bullet with one nice big lube groove doesn't need it. But several bullet designs, especially rifle bullets, can have 3, 4 or even 6 small sharp-edged lube grooves that need some tin to get perfect fill-out of the bullet in the mold.
Something like these 358627s will benefit from at least 1-1/5% tin in the mix.
