454 - do you pre-heat your RCBS mould? For iron moulds, like RCBS or Lyman, I dunk them right into the melt. You will need to do a bit of experimenting, but here are some numbers to get you started if you want to try it:
Lyman 40 cal 400 gr Snover bullet (#410663, I think) - In a 20-lb pot, I get the melt (20:1 lead to tin) to 800 degrees, then dip the front edge of the mould into the melt for 1 minute and 45 seconds. Only about 1/3 to 1/2 of the mould is actually submerged in the molten metal. This will drop the temperature of the melt to about 775, and that's where I like to cast with this alloy and that mould. I usually toss the first bullet back in the pot, but starting with the second or third bullet, they're perfect.
I used to dunk the bare mould right into the melt, but eventually you'll get lead up into the handle screw holes and other places. So now I cover the bottom and sides of them tightly with heavy duty aluminum foil (it tears easily so be careful).
Obviously, if your pot is smaller than 20 lb., the alloy temp will drop a lot faster. You may need to get things hotter to start with.
This doesn't warp the mould at all. These bullets are used for match shooting in my Sharps rifles - and if the moulds warped, I'd know it.
If you are using an alloy like Linotype, you don't need to get it anywhere near 800 degrees. I think you could cast with Lino at around 650?
