08 November 2009
Agreed, heat treating is to increase velocity, there's no other reason. Veral Smith looked into it in some depth in his book “Jacketed Bullet Performance with Cast Bullets." I looked at it myself in a minor way so I'm no authority, but this is what I know for what its worth.
Velocity of cast bullets is limited by leading. There are various factors in leading or prevention. Those are velocity, bore smoothness, alloy hardness and lube quality. Hardness is a material factor. Pure lead is 5 BHN. Typical handgun bullets are 12 - 15 BHN. Linotype ( that's the 4/12 linotype we usually mean ) is 22 BHN. Lead alloys can be harder, like some of the monotypes with higher antimony content.
The attraction of heat treatment is that it gets high hardness, up to 35 BHN, without the need for a lot of antimony. However, the seldom mentioned glitch is that it needs arsenic. Not just any amount but about 0.25%. Much less doesn't get results and much more results in embrittlement and possible bullet fracture. In the past wheelweights have heat treated successfully because they contained just the right amount of arsenic. Don't know whether that can still be relied on, but there used to be a specification of 0.50% tin and 4% antimony, and I can tell you that wheelweights don't meet that spec these days in my part of the world.
There is no guarantee that any other alloy will contain the right amount of arsenic, and the only way you can guarantee that is to blend your own from virgin materials.
Antimonial alloy work softens, so yes, sizing a heat treated bullet can be expected to soften it. The answer is to size them before heat treating them.
My feeling is that it just ain't worth the trouble. Rather stick to a good alloy you blend yourself and ensure consistent hardness that way. Bullets cast with enough antimony for rifles are still dirt cheap compared with jacketed, and the cost of a decently hard bullet is not much more than one that's too soft.
If we want to keep our cast bullets economical the best place to look is the high cost of gas checks.