Further note about PP bullets.
Now your experience might differ, but we (bro & I) that quite soft bullets worked better than even 20:1, lead to tin worked. We were using black powder and not smokeless so that might have been the reason.
I would suggest 30:1 to 40:1 being better, but of course, that depends on the rifle and what IT prefers.
Alloy is just another variable in testing, as-is the wads used.