ghostrider wrote:Are there rules of thumb (calibre, intended velocity, alloy, etc) to determine whether cast bullets should
be water quenched (hardened) or not?
Yes, Richard Lee 's Modern Reloading
contains tables that show the relationship between pressure and required lead alloy hardness.
Load tables that list the bullet weight, powder charge, primer and COAL, also list the resulting cartridge pressure and velocity.
So, simply dial in your load and note the published pressure. Then you look up the required hardness. You play with your bullet alloy or water quench to reach the desired hardness.
For example, here is a .38SP load:
90 GR. CAST LRNFP IMR Trail Boss .359" 1.325" 3.0 740 7,800 PSI
At 7,800 PSI, you should have very soft alloy.
Conversely, there are published loads for .454 Casull that are in the 54,000 PSI pressure range. Obviously, it requires a much harder alloy (or gas check).
If you really want to get precise, you'll need the Lee lookup table and a means of testing your alloy hardness. I use the Lee hardness tester.
Otherwise, it is trial and error. For higher pressure cartridges like 9mm or .40 S&W or above, cast and load a couple of hundred without water quench and a couple of hundred with water quench. Run them through your gun as separate trials, look at the leading, and choose the one you like.
Also, if you are trying to dial in a load, leading may have nothing to do with the hardness as most of the time it is a sizing issue rather than a hardness issue. Taking the time to slug your barrels and/or chambers so you know your actual size requirement is time well spent.
When I water quench, I simply use an aluminum roasting pan on the table beside my pot. That water isn't magically going to jump from the pan and submerge itself in my melt.