It is my understanding that a shotgun that can fire 410 gauge shells as well as 45 Long Colt.
NO!!!!!!!
You have it exactly backwards!!!
The combo caliber guns .45Colt/.410 (note how its written) the Judge, etc. Even the T/C Contender are .
45 caliber guns, that happen to be able to fire the .410 shotshell. NOT the other way around.
You can make a long gun to fire .45Colt/.410, and as long as it meets the Federal (& state, if any) requirements for length, (both barrel and overall length) it can be rifled, or smoothbore, and be legal.
Rifled, your performance with .410 shot shells is going to suck. Even though the .410 is undersize for the bore, it is close enough that the rifling will have a spin effect on the shot, while gives a "donut" shaped pattern (hole with no shot in the middle). Legally it will be a rifle.
If you make it smoothbore, then legally its a shotgun. And, since you get rather poor accuracy from a smoothbore .45 using .45 Colt ammo, there's no point. A regular .410 shotgun is better.
The combo handguns are all rifled, they have to be, in order to meet federal law and not be considered "sawed off" shotguns.
Don't shoot .45 caliber ammo (any) in a .410!!!
Also, due to slight dimensional differences, you need a different chamber to shoot .45 Colt and .45ACP. Note that the Ruger Blackhawk convertible .45Colt/.45ACP uses two different cylinders, one for each round.
Also, FWIW, a .410 shell will not chamber in a standard .45 Colt chamber. The .45 chamber has to be cut to allow the extra length of the .410 shell, otherwise, it won't fit.