Here are a few excerpts from,
From the NRA Publication "Cast Bullets" by Col. E.H. Harrison USA (retired)
The Article "The Tin In Your Cast Bullets" by Dennis Marshall, Pg 130
During casting, the tin portion of a lead/tin alloy divides; part of it is retained within the lead grains which make up the bulk of the bullet while the remainder segregates to the grain boundaries as lead/tin eutectic. While the lead grains are still hot, immediately after casting, they dissolve a large amount of tin, However, the solubility of tin in the solid lead decreases with temperature, and when a bullet cools its lead grains are left with an excess of tin which must be rejected from solution. In the process of being rejected, tin is trapped inside the lead grains and is unable to make its way to the grain boundries, Thus constrained, tin forms particles or precipitates in the midst of the grains.
Permanent deformation, such as engraving a bullet by the rifling, occurs by movement of imperfections through the grains. Lead can be made stronger if the movement of these imperfections can be blocked. Tin precipitates act as barriers to the movement, and thereby help the bullet resist deformation.
Precipitation hardening depends on a mumber of complex factors. An important one of these is the strength of the precipitates; stronger precipitates harden better. Unfortunately, the strength of tin is on a par with lead. Imperfections moving through lead grains meet relatively weak tin obstacles. Strength imparted is due to other factors such as the shape of the precipitates.
Tin plays a considerably different role in type metals; improving not only the casting properties but also contributing to the strength. Unlike lead/tin alloys, the microstructure of type metals does not contain weak tin precipitates. Instead, tin combines with antimony during solidification to form an intermetallic compound, SbSn, which is approximately the same hardness as antimony.
SbSn is one of three possible constituents in the microstructure of type metals; the other two are lead and antimony. For alloys like linotype, part of the strength is attributed to precipitation hardening of the lead constituent; i.e., hard antimony and SbSn precipitates form within the lead to strengthen it. Compared to tin, these hard precipitates are very effective in hardening lead. The remainder of the strength is imparted by large antimony and SbSn particles.
Within certain contraints, raising the tin and antimony content increases hardness. However, the greatest hardness increase per unit alloy addition occurs when antimony and tin are added in equal proportions. The effect is most noticeable when the combined antimony and tin content exceeds about 12%. The microstructure of such alloys consists of precipitation hardened lead and hard SbSn particles; there are no antimony particles present. If cost were no object, the tin content of linotype could be increased to 12%, matching the antimony content, and increasing the hardness about 2 BHN.
For tin/antimony ratios greater than one, SbSn is the principal hardening agent, but a fraction of the tin will precipitate as in lead/tin alloys. As the tin content increases, so do the tin precipitates with no corresponding improvement in strength.
So, how much tin to add to your bullet alloy? It's generally accepted that todays WW contains at best 3% antimony so if you add more tin than 3% your actually softening the alloy and increasing its rate of age softening.
Hope this helps explain it.
Rick