It is a (dangerous) urban legend to believe that cap and ball revolvers are not firearms.
Carry it concealed unlawfully, and you'll be charged just as quickly as if it were a Glock. This question comes up all the time. That federal designation of "firearm" and "non firearm" apply only to regulation of sales, as I understand it.
I'm sure someone will chime in after me with the proper Federal Statute, but I've seen it quoted time and again. It's a mistake, now bordering on Urban Legend, that black powder revolvers are not legally "guns" so you can carry them concealed without a permit, or are not required to declare them in your check-in airline luggage.
Get caught doing the above and you'll find yourself in front of a judge just as surely as if it were a .38 snubnose. People want to argue the point until Doomsday, but what they need to do is call the ATF, their local prosecutor and the local sheriff to see what they say about it.
When I was a reporter 20 years ago in Idaho, a convicted felon was charged with possessing and carrying a concealed handgun. It was a Ruger Old Army in a shoulder holster. In no uncertain terms he was told by the sheriff, prosecutor and judge that if it used a substance to propel a projectile across the room, it was by definition a gun. This included air guns, by the way.
Others will tell you that black powder revolvers are not guns by definition. If you heed their advice, ask them to contribute to your bail. You'll need it.