My uncle has cast up a bunch of 125 gr .358 RNs so I can load them for his .38 Long Colt and my Model 66 S&W. I know that I need to size and lube them before I load them, and I was going to buy one of those Lee luber/sizers that you put in your reloading press. My question is this, do I need one for .358 dia or .357? I know cast bullets are generally .001 bigger than the jacketed bullet for the same caliber, but would it hurt anything to use a .357 sized cast bullet? Reason I ask, I can get a .357 sizer for cheap, but the .358 will probably cost me more since I have to order it. I'm thinking since a Ruger Blackhawk can be bought with a cylinder that shoots 9 mm (which are only .355 for jacketed or .356 for cast) it really can't hurt anything to shoot a .357 cast bullet in a bore that is made for a .357 jacketed bullet. Any thoughts on this?

Most 38s/357s like bullets .3585 to .359. A buddy of mine uses .360. The important number for revolvers is the cylinder mouth's diameter not the barrel. If the bullet is to small you will get leading and poor accuracy.
Just because a cartridge can be fired in a firearm does not mean it is accurate. If you want to spend the money a gunsmith can make you a cyclinder that holds 32 caliber rounds for your 357 Ruger. The bullet will rattle down the barrel and will eventually hit the ground.

Jim

First order of business, check the bullet diameters as they are before sizing they may be undersize right from the begining, probably not but you never know. It would be nice to know the diameter of the cylinder throats but the tools to do that aren't always available. Because your question is about which sizer to get I would suggest the .358 diameter, better to be a little big than to small, neither is real desirable but to small all but garantees poor accuracy and leading. There are work arounds that will work but require lenghty explanations. The reason the Ruger with interchangeable cylinders works reasonably well is because slightly undersized jacketed bullets may catch enough rifling to be fairly accurate. Not so with cast bullets if they don't get a good solid grip on the rifling they will just strip off and cause leading of the barrel.

Thanks for the explanation, I was thinking that the larger bullets would leave more lead because the rifling would take a bigger bite out of them, but it makes sense that undersized bullets might "skid" across the rifling a bit and cause heavier leading as the bounce down the bore. I will try to get the .358 sizer. My uncle said that he used a .358 mold so the bullets shouldn't be undersized at all, but I will check them with my digital caliper when I get them.