how fast can i drive a 255 grain slug in my 45 colt blackhawk before ihave to worry about leading?
thank ya,

You didn't say what hardness your bullets are, but if you want to hang on to it, air cooled wheel weights should put you up to 2000 fps or so before it leads. This assumes proper throat sized bullets. For regular pistol velocities, if the bullet is the proper size and hardness ( not too hard!), you will be able to shoot to factory velocities easily.

sorry waksupi,
their cast from w/w.cylinders all mike at .451 and i size to .451.

If all cylinders are .451, size to .452 or .453.

What is the groove diameter of your barrel? I sluged my Ruger and it was .452. If yours is the same and you have .451 throats I would suspect that leading would happen at a lower velocity. I opened up my throats to .4525 and now size to .452.

I agree, size larger and maybe think about opening up the cylinder throats if you don't get good accuracy or experience leading. You should be good for any velocity that you can safely achieve.