Some US Gun Laws by state have definitions for antique firearms while others do not. However, the definitions of antique guns are almost the same and vary only slightly. For the purpose of arriving at a common understanding of this type of firearm, the definition given by the National Firearms Act uses the term "Antique Firearms", which points to any form of firearm that is not intended or redesigned to utilize rim fire or conventional center fire ignition with fixed ammunition. The Act also adds that it should be manufactured before or within 1898.
These types of firearms do not use fixed ammunition or ammunition no longer made in the US and not readily available within the conventional channels of commercial trade based on US Gun Laws. If one or more of the stated elements of an antique firearm is missing (for example, if an antique gun can already use ammunition which is readily available commercially due to modification), then NFA regulations would already apply based on the ATF website. But it should be noted that despite NFA regulations that would already apply to a pre-1899 gun, its classification as "antique" remains.
Canadian Law provides that antique firearms can be imported if they are manufactured prior to 1898 and comply with certain criteria. For additional information see: laws-lois.justice.gc.ca The UK, Australia and many other countries have similar requirements and most allow antique guns to be imported. Check with your elected officials or online for clarification.