I have a T/C Contender with the 10" 410/.45Colt barrel.
I think I know wht y'all are going to say, but is there any reason I could not shoot the .45ACP in this gun? After all, isn't .45 always a .45? The chamber is completely smooth into the barrel/riflings.
After all, it is designed to shoot the much longer 410 shell as well as the .45 Colt.
I wouldn't try it through a revolver for obvious reasons, but the Contender is a break action.

No- for several reasons.

-The .45 colt uses a .454" bullet while a .45ACP uses a .451" bullet- the bullet would still be undersized but accuracy would suffer.
-The ACP headspaces on the mouth while the .45 Colt headspaces on the rim. You could potentially load .45 ACP rimmed cartridges, but that would negate any reason to shoot the .45 ACP in the gun instead of the .45 colt.

I'm such an idiot!
I just noticed the rim difference between a semi-auto (.45ACP)load and a revolver (.45 Colt). The .45ACP has a rim design that would allow it to slide all the way through the barrel!

Actually about any modern 45 Colt uses .451 to .452 bullets just like the 45 ACP. The .454 hasn't been used in years.

The ACP won't fire in your 45 Colt Contender though. About the only way to fire them in the same gun is by using different barrels (like the Contender can) or interchangable cylinders (like the Ruger Blackhawk).

This might be a stooopid question ,But could you use some kind of moon clip?

Not a stupid question at all. It would work provided the firearm had enough "space" for the clip. That's where the problems lies though. Most 45 Colt designed firearms wouldn't have enough room for a moon clip.