All antiques can be shipped, if one complies to all the regulations for C&R FFLs. As I can understand, the problem is that the Post Office does not allow any fixed ammunition handgun shipped without an FFL who has registered with USPS.
The following is the statement from USPS regulations:
a. Firearms meeting the definition of a handgun under 431.2 and the definition of curios or relics under 27 CFR 478.11 may be mailed between curio and relic collectors only when those firearms also meet the definition of an antique firearm under 431.3.
The definition of an antiques by the USPS, 431.3, is tough to understand, but is below:
431.3 Antique Firearm
Antique firearm means any muzzle loading rifle/shotgun/pistol, which is designed to use black powder or a black powder substitute, and which cannot use fixed ammunition (except those that incorporate a firearm frame or receiver, any firearm which is converted into a muzzle loading weapon, or any muzzle loading weapon which can be readily converted to fire fixed ammunition by replacing the barrel, bolt, breechblock, or any combination thereof); or any firearm (including those with a matchlock, flintlock, percussion cap, or similar type of ignition system) manufactured on or before 1898, or any replica thereof, if such replica:
a. Is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition.
b. Uses rimfire or conventional centerfire fixed ammunition that is no longer manufactured in the United States and is not readily available in the ordinary channels of commercial trade.
§ 478.93 Authorized operations by a licensed collector.
BATF Regulation Subpart B § 478.11
1. Antique firearm.
1. a. Any firearm (including any firearm with a matchlock, flintlock, percussion cap, or similar type of ignition system) manufactured in or before 1898;
2. b. Any replica of any firearm described in paragraph (a) of this definition if such replica:
1. 1.Is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition; or
2. 2.Uses rimfire or conventional centerfire fixed ammunition that is no longer manufactured in the United States and that is not readily available in the ordinary channels of commercial trade; or
3. c. Any muzzle loading rifle, muzzle loading shotgun, or muzzle loading pistol that is designed to use black powder, or a black powder substitute, and that cannot use fixed ammunition. For purposes of this paragraph (3), the term "antique firearm" does not include any weapon that incorporates a firearm frame or receiver, any firearm that is converted into a muzzle loading weapon, or any muzzle loading weapon that can be readily converted to fire fixed ammunition by replacing the barrel, bolt, breechblock, or any combination thereof.
2. Curios or relics. Firearms which are of special interest to collectors by reason of some quality other than is associated with firearms intended for sporting use or as offensive or defensive weapons. To be recognized as curios or relics, firearms must fall within one of the following categories:
1. a. Firearms which were manufactured at least 50 years prior to the current date, but not including replicas thereof;
2. b. Firearms which are certified by the curator of a municipal, State, or Federal museum which exhibits firearms to be curios or relics of museum interest; and
3. c. Any other firearms which derive a substantial part of their monetary value from the fact that they are novel, rare, bizarre, or because of their association with some historical figure, period, or event. Proof of qualification of a particular firearm under this category may be established by evidence of present value and evidence that like firearms are not available except as collector's items, or that the value of like firearms available in ordinary commercial channels is substantially less.