From the ATF:
http://www.atf.gov/publications/download/p/atf-p-5300-4.pdf
Page 160:
"8. ANTIQUE FIREARMS
Under section 921(a)(16) of the GCA,
the term antique firearm means:
(A) any firearm (including any firearm
with a matchlock, flintlock, percussion
cap, or similar type of ignition
system) manufactured in or before
1898; or
(B) any replica of any firearm described
in subparagraph (A) if such
replica—
(i) is not designed or redesigned
for using rimfire or conventional
centerfire fixed ammunition, or
(ii) uses rimfire or conventional
centerfire fixed ammunition which is
no longer manufactured in the
United States and which is not
readily available in the ordinary
channels of commercial trade; or
(C) any muzzle loading rifle, muzzle
loading shotgun, or muzzle loading
pistol, which is designed to use
black powder, or a black powder substitute,
and which cannot use fixed
ammunition.
For purposes of subparagraph (C), a
muzzle loading rifle, shotgun, or pistol is
not an “antique firearm” for purposes of
the GCA if it incorporates a firearm
frame or receiver, if it is a firearm which
is converted into a muzzle loading
weapon, or if it can be readily converted
to fire fixed ammunition by replacing the
barrel, bolt, breechblock, or any combination
of such parts.
...
An antique firearm as defined in both
the GCA and NFA is exempt from all of
the provisions and restrictions contained
in both laws. Consequently, such an
antique firearm may be bought, sold,
transported, shipped, etc., without regard
to the requirements of these laws."
Again, this applies at only the Federal Level. State laws may prohibit the possession of antique and/or black powder firearms where they are not prohibited by Federal law.