First, you would be better served posting your question in the
Handloading & Reloading section of this board.
Next, I'm far from an expert, but here's what I've figured out through reading, since I started loading this caliber: Older Colt (the manufacturer, not the caliber) firearms (real old; pre-WWII, etc) had larger (.454") bores and appropriately sized chamber throats. This is why they still make .45 Colt bullets sized to this larger diameter.
Modern .45 Colt (caliber) guns have .451" or .452" bores and shoot .451" to .452" bullets. Most jacketed bullets I've seen have been .451". Lead are available in both sizes. You must slug your bore to determine it's actual size.
Modern
Ruger .45 Colt guns generally have
undersized chamber throats (.450") which squeeze the bullet down to below bore size, which negatively affects accuracy, as well as increasing pressure in the chamber. You can have this corrected by many places, but I recommend Cylindersmith.com; The guy that runs that business is a "regular" over on the RugerForum.com site.
Properly sized lead bullets are generally preferred for accuracy sake, but you also have to find the right powder charge for that gun/bullet combination to keep leading to a minimum.
My thinking is that you should
never try to use the .454" bullets in a modern .45 Colt unless you know your chambers and bore are
oversize. Doing so is just asking for an overpressure situation. (Caveat: Safety is very high on my priority list.)
You need to slug your bore and chambers to find out their exact size. Ideally you will have something close to: .4525" chambers and a .451" bore. You would then use a .451" bullet because that would seat tightly in your .451" bore. You
could use a .452" bullet in this situation, but it would cause an (acceptable?) pressure spike. I would
not want to use a .454" bullet in this situation.
Slugging will give you a "relief image" of the bore. The bore grooves will cause high point ridges on the slug. Measure across these ridges and you'll have the exact width across the grooves, which is your bore diameter. When measuring the chamber throat diameter obviously there are no grooves.
-- Sam