Quoted:
A rifled shotgun barrel is still a shotgun barrel .......

Quoted:
Remember you can only have a .50 cal or less in a rifle. Only shotguns and NFA regulated guns have larger.

Quoted:

The NFA definition of a rifle does not say anything about caliber.

The 50 caliber limit is only used to define a destructive device  unless the weapon has a sporting purpose. ATF says most shotguns have a sporting purpose.   In fact this was used to ban shotguns that the ATF didn't like in the 80s and 90s, they simply re-classified them as destructive devices instead of Shotguns.

Quoted:
Cutting down a rifled shotgun barrel... I assume I can cut it to 16 inch (instead of 18) since it's rifled, correct? The gun still needs to be 26 inch overall too, correct?

Destructive device. (a) Any explosive, incendiary, or poison gas (1) bomb, (2) grenade, (3) rocket having a propellant charge of more than 4 ounces, (4) missile having an explosive or incendiary charge of more than one-quarter ounce, (5) mine, or (6) device similar to any of the devices described in the preceding paragraphs of this definition; (b) any type of weapon (other than a shotgun or a shotgun shell which the Director finds is generally recognized as particularly suitable for sporting purposes) by whatever name known which will, or which may be readily converted to, expel a projectile by the action of an explosive or other propellant, and which has any barrel with a bore of more than one-half inch in diameter; and (c) any combination of parts either designed or intended for use in converting any device into any destructive device described in paragraph (a) or (b) of this section and from which a destructive device may be readily assembled. The term shall not include any device which is neither designed nor redesigned for use as a weapon; any device, although originally designed for use as a weapon, which is redesigned for use as a signalling, pyrotechnic, line throwing, safety, or similar device; surplus ordnance sold, loaned, or given by the Secretary of the Army pursuant to the provisions of section 4684(2), 4685, or 4686 of title 10, United States Code; or any other device which the Director finds is not likely to be used as a weapon, is an antique, or is a rifle which the owner intends to use solely for sporting, recreational, or cultural purposes.
and
Short-barreled shotgun. A shotgun having one or more barrels less than 18 inches in length, and any weapon made from a shotgun, whether by alteration, modification, or otherwise, if such weapon as modified has an overall length of less than 26 inches.
and
Shotgun. A weapon designed or redesigned, made or remade, and intended to be fired from the shoulder, and designed or redesigned and made or remade to use the energy of the explosive in a fixed shotgun shell to fire through a smooth bore either a number of ball shot or a single projectile for each single pull of the trigger.