I forget where I learned this, but I think that any gun using a design prior to 1865 is not considered a firearm under the ATF's rules. Does anybody know about this for sure?
Regulated guns under the 1968 Gun Control Act are anything made after 1898 or
using commercially available ammunition. This only regulates the interstate transportation of firearms. There are a number of federal, state, and local laws that regulate possession, transportation, and use.
The definition of a firearm can be different under each law. For example, with regard to possession, the Federal government and the Commonwealth of VA specifically exclude matchlocks, flintlocks and percussion ignitions by name as
antiques(meaning not regulated by the applicable laws). Otherwise, you must be 18 or older to purchase or possess a firearm. You can purchase or possess an antique (all muzzle loaders) or air gun (not a firearm) at any age.
However, VA does regulate the transportation of flintlocks and percussion guns, with different rules for each, as to whether they are considered loaded or not. You may not transport a loaded firearm, antique or not, in the state of VA unless you have a concealed carry permit. Also, with regard to possession of a firearm by a felon, ALL firearms are included. That means that if you are convicted of a felony in VA, you loose your antique collection as well as any modern firearms! These are the regulations that affect me. So, these are the ones I am familiar with. The laws pertaining to you may be different.