Is it a "conversion" utilizing something like a Kirst or similar . . . or is this a "conversion" that was made as a conversion?
If it is a pistol that was made as a "conversion" such as a Uberti Richards & Mason, etc. . then the bore should be .452. If not and it's a conversion made with a C & B revolver and a conversion cylinder, then you're going to have to use a slug that is hollow base or "heeled" so that it expands as it enters the bore.
In handguns, if shooting cast, it's common to slug the bore. Use a soft lead sinker or such, oil the bore well and carefully drive the slightly oversize sinker through the bore with a brass rod or similar. Then measure with a micrometer. A set of vernier calipers/digital calipers can give you a "rough" measurement but not an accurate measurement.
On cast, usually .001 to .002 oversize of the bore measurement works well. As mentioned, it does no good to try and put a large bullet through a smaller cylinder throat and then expect it to shoot accurately in a bore larger than the throats are as the bullet is swaged down before it gets in to the bore - that will give you poor accuracy and usually "leading" to the bore.
If your bore slugs larger than the throats, it may be necessary to have the cylinder throats honed or opened up to the bore size or just a tad larger like .0005 or so.
I have a .454-190 Ideal mold that drops larger than .454 - that's not uncommon with molds depending on the alloy you use. For my BP cartridges, I use soft lead - the same pot that I use for casting round ball and minie balls. I then finger lube with my BP lube at the time I seat the bullet.
If you mold drops heavy (diameter wise) - you can always get a Lee push through die that fits in your press to size them down. They make a .452 but it's not hard to polish them out to a larger diameter such as .453 or .454. A dowel with a slit cut in it and a strip of fine emery paper slipped in and wrapped around the dowel, a drill or drip press and some oil will do it just fine. Lee push through sizing dies can often be "off" - I've had several. You can get them from Lee dealers - I like Titan Reloading as they have excellent prices and Dennis gives excellent service. Polish and then size a bullet to check your progress as you are polishing the die out so you don't get it too large. If you end up using a larger bullet, you may have to adjust the size of your expanding plug in your expanding die to allow the correct neck tension as you seat.