Great point on sizing Edward. Certainly overlooked by us when posting about the alloys. One point leads to another!
Something else to consider when casting and sizing, is that different alloys will shrink to different diamaters whe cooling. Generally the harder the alloy, the more it will shrink.
Several custom mould makers will cut a mold to your specs for the alloy you wish to use so that you don't have to size. It can be a great help in casting accurate bullets for rifles, but I've always just used the standard RCBS and Lyman molds for my pistol shooting.
Most of my older Lyman 30 calibre molds (circa the 1960s) throw bullets several thousands over what I need and sizing is the only option. I'd rather not size a bullet too much so I can reduce that to an extent with a hard alloy. Generally I size to about one to two thousands over bore size.
I use an RCBS lubrisizer that I've had since about 1975. It was sure a huge step forward for me when I bought it and it is still pumping out sized and lubed bullets. Over the years I've collected sizing dies for it so that I can size up to or below most of the popular calibres by several thousands of an inch. Ie. I can size 30 calibres (depending on the mold) from .308 up to .315 depending on the bore diameter. That pretty well covers US 30 cals as well as the British, Russian and Japanese 30 cals that can run well over .312 in military rifles.
This was mentioned before, but If using a softer alloy, you can harden the final product some by quenching the hot bullets from the mold into cold water.
Most of the hard cast commercial bullets that I buy are usually .001" over standard bore size for their intended calibre. Unless you are shooting an older pistol with out-of-spec bore and throat diamaters they will probably work fine without further ado. As I mentioned in my first post, they do resist obturation due to their hardness, so you may need to shoot them faster than intened. Quite often though, that will reduce leading with them, particularly at the throat or forcing cone. Counterintuitive I know, but that's been my experience since they became common 20 some years ago.
So how do you go about finding out your bore size? Soft lead bullets (sometimes muzzle loading balls if the right diamater) slightly over your bore size are used. To measure the result after slugging your barrel with one, you'll need a good micrometer. Mine are old school and read via etched on scales, but newer ones will read to .0001 with a digital read out. Note to self: Need one of those; eyes aren't what they used to be. Oh yeah, read the groove diamater for the datum needed, not the land diamater, although it doesn't hurt to know both.
Barrrels with an odd number of grooves such as Trapdoor Springfields (3 groves) and Lee Enfields (5 grooves) are difficult for me to measure. Sometimes it is just experimentation that will find the solution for them. I don't think that I've ever run accross a US made pistol that I could't measure accurately.
Please don't let all this minutia drive you away from cast bullets. Once you find a good load, you'll never look back and may just enjoy finding new challenges. Getting my old antique firearms to the range and shooting them as well as all the new guns is quite the hoot for me. Cast bullets allow me to do that.