Differences between 30:1 and 15:1?

[Naphtali]

What is the difference in BHN, if any, between 30:1 and 15:1 lead:tin mixture?

What is the difference in diameter of [front] driving band, if any, between the two mixtures casting any standard 45 Colt 260-grain WFN bullet?

[Dusty Bannister]

Probably none.
http://castboolits.gunloads.com/showthread.php?50283-Does-adding-tin-affect-boolit-diameter

[Larry Gibson]

The 15 - 1 has twice the tin in it and will withstand higher acceleration. I use 16-1 for magnum level ha gun loads of 1350+ fps, PB cast from 1200 - 1600 fps and GC cast to 2000 fps where expansion is wanted in lieu of softer lead- tin alloys. Fill out can run .0005 higher depending on caliber.

Larry Gibson