Mine is a Savage Striker S.S. in a .243 Win. So it is in between a Rifle and a Pistol with a thumbhole grip.
I dont know if its a Pistol or Rifle in USPS 's eyes?
fir them I think they will be figuring barrel longer 16.0 inches or longer and a shoulder stock it is a rifle.
barrel less than 16 inches it is not going to ship USPS and should be done through a dealer to play it safe.
there are many that only fit the definition "firearm" play it safe on those and treat them like a pistol a "firearm" may be a shockwave shotgun or a shorter than 16 inch barrel with a pistol brace but that still makes >26 inches in length.
but since USPS quotes 431.2 I think it would consider most "firearms" as handguns
although the >26 inches for most common purposes means not or less concealable they don't necessarily have the understanding that not every gun fits rifle , shotgun or handgun, so play it safe.
431.2 Handguns
Pistols, revolvers, and other firearms capable of being concealed on the person (for example, short-barreled shotguns and short-barreled rifles) are defined as handguns. The following definitions apply:
Handgun (including pistols and revolvers) means any firearm which has a short stock, and is designed to be held and fired by the use of a single hand and subject to 431.1, or a combination of parts from which a handgun can be assembled.
Other firearms capable of being concealed on the person include, but are not limited to, short-barreled shotguns and short-barreled rifles.
Short-barreled shotgun means a shotgun that has one or more barrels less than 18 inches long. The term short-barreled rifle means a rifle that has one or more barrels that are less than 16 inches long. These definitions include any weapon made from a shotgun or rifle, whether by alteration, modification, or otherwise, if such a weapon as modified has an overall length of less than 26 inches. A short-barreled shotgun or rifle of greater dimension may be regarded as nonmailable when it has characteristics to allow concealment on the person.