That's right. Here is the specific law:
18 USC § 2332a(C) Definitions. For purposes of this section
. . . . . .
(2) the term “weapon of mass destruction” means—
(A) any destructive device as defined in section 921 of this title;
18 USC § 921 - Definitions
4 The term "destructive device" means—
(B) any type of weapon (other than a shotgun or a shotgun shell which the Attorney General 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 subparagraph (A) or (B) and from which a destructive device may be readily assembled.
There is however, this caveat:
The term “destructive device” 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 signaling, 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; or any other device which the Attorney General 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.
So in short, it's up to the AG to determine whether you intended your potato gun to be a weapon or a recreational device.