"A Colt's Mfg. 12 ga. side-by-side with external hammers"
I'm not 100% sure about this, but I believe that ALL Colt hammer double shotguns were made ONLY for blackpowder and all had Damascus twist barrels.
A hammer gun is the Model 1878, a hammerless gun would be an 1883.
Another reason to NEVER fire modern ammunition in any of these guns without a good going over is that the chambers on the 1878s were almost all 2 1/2", which was the standard length for 12 gauge shells until well into the smokeless powder era.
Putting a 2 3/4" or worse, a 3" shell in one would almost undoubtedly bulge the barrels and could very well blow them at the chamber.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.