The overall amount a bullet can be sized up or down is dependent on the design of the bullet you are trying to change the size on.
It has nothing to do with a gas check. And it has little if nothing to do with Tin in the bullet. Yes a harder(in Brinell hardnss)bullet will be tougher to size because of the nature of the alloy.
Gas checks are used when you are using "heavy" loads and you do not want the base of your bullet to be flame cut. They are totally unnecessary for bullets in lower velocitiy situations. GC's will also prevent leading in the higher velocities. IMO above 1600-1800 fps. I have pushed WW to 1800 fps without leading if the bullet is sized properly.
Sizing is more important to leading that GC's for most cast bullet shooting.
Tin is used to make your alloy "flow" better. It makes it easier for the alloy to flow into all the crevices. Tin is not used to make a bullet "harder". And you can't keep adding Tin to an alloy. About 5% Tin is optimum. Anything over 5% is just a waste of your Tin.
Antimony is what is used to make your alloy harder. That is why linotype is so popular to casters. It has a high % of antimony.
I never size more than .002. Possibly .003 for a bullet that has a special design. Antything more, then get the proper mold for the job. Tom.
