Boats,
Could you measure any appreciable size difference in your .32 cal. bullets when you went to 1-20?  Bob

re: size   I've found that mould and alloy temperature can easily vary the diameters.  (as well as how fast it's filled or distance under the bottom pour furnace).

My question is one of accuracy.

Of the alloys, which is easier to shoot accurately?

(For me, COST of tin is not an issue as it's cheaper than lead.

Precision casting is easier with a bit of antimony - but the curse of it is hardness and harder to achieve accuracy.

CW if your are using a long spitzer design, the extra strength in the richer tin alloy helps to minimize nose slump. What I mean by this is that long unsupported nose section due to the centrifugal force encountered will slump or maybe a better word to help visualize what is taking place would be pushed out of being central with the bore creating an imbalance to the bullet. With these designs I have found the hardest lead and tin alloy that does not create a leading issue will indeed provide better accuracy and 1-16 has worked very well for me over the years. A close friend did some catch box testing quite a few years back and found nose slumps from 5 to 12 thousandths with if memory serves me right 1-25. His testing is what prompted me to try harder lead and tin alloys and they have worked very well for me. Bullet fit might become a tad more critical due to possibly less bullet bump up. The possibility of less bullet bump up is pure speculation on my part. Another thing to take into consideration is what is happening to the bullet when it fills the grooves and changes it's original form by displacement of the alloy. We also want this taking place in such a manner as to not have the nose slump from again being central with the bore. This is where having having all of the lube grooves equally filled also comes into play. If one side of the bullet is lacking more lube than the other, the lacking side will displace more alloy into the groove creating a bullet imbalance and possibly inducing more nose slump. I hope you find this information helpful and that I did not make it to confusing.

CW it surely is worth the effort and I wish you the best.

Dave great information, we only differ in my thinking from my extensive testing that 1-18 would probably make for the best all around alloy in regards to long pointed bullet use. The only down side I have found when sharing my findings is the unwillingness to pay the additional cost of the tin for the increase in accuracy. I hear I can't get my rifle to shoot up to par all the time, so I share what little I really know and the bottom line quite typically is they really don't want to. Either due to a little additional cost such as the tin or the time and effort it actually takes to try all of possible improvements they have yet to try. One fellow out my way thinks if he buys enough rifles at some point in time one of them is bound to put him in the winners circle. In reality all of the ones he has purchased to date would probably do exactly that with a little time effort and possibly a minor additional cost for the correct alloy.

when is tin cheaper than lead?

Only when you can pick up lead solder waste at the radiator shop for nothing. 

But virgin "Tin" here costs about 10 times the price of lead. 

Who said the Tin was cheaper, it is about 5 times more expensive than the lead and the reason some shy away from the benefits of a richer Tin alloy.