Yes; other than what Aguila and Armorer already stated, be aware that the postal definition of a "short-barreled rifle" and/or "firearm... capable of being concealed on the person" is subtly different from the NFA definition and other familiar federal definitions in an important way.
The postal regs define a "short-barreled rifle" as...
...any weapon made from a rifle (by alteration or modification) resulting in an overall length of less than 26 inches.
IOW the rifle must be at least 26" long as mailed. Do NOT disassemble it to fit into a smaller package; your attempt to save on shipping costs could land you in hot water! This is particularly important with modular firearms such as AR's.
"..made from..." clearly means the rifle (or shotgun) was originally 26".....and was altered to be less than that, and OAL is not measured when dissassembled.
While the USPS prohibition of "short barreled rifle" isn't word for word identical to ATF regulations, it clearly does not prohibit shipping a dissassembled firearm........if it did there are 10,000 violations every day as Remington, Browning, Benelli, Beretta and dozens of other firearm manufacturers box their shotguns and many rifles in dissassembled condition.
Further, Federal law dictates how firearms are measured for overall length........and that is ASSEMBLED. If the firearm has a folding stock, ATF measures the OAL with stock extended.
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