This letter is relevant as to why, with an overall length greater than 26", it might not be an "any other weapon" :
http://www.nfaoa.org/documents/PistolGrippedShotgunLike.pdf
But since it is not a shotgun, why is it not a "Destructive Device"
(4) The term “destructive device” means—
(A) any explosive, incendiary, or poison gas—
(i) bomb,
(ii) grenade,
(iii) rocket having a propellant charge of more than four ounces,
(iv) missile having an explosive or incendiary charge of more than one-quarter ounce,
(v) mine, or
(vi) device similar to any of the devices described in the preceding clauses;
(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;
