Rifle-caliber and pistol-caliber are diversions from the issue, given that it is entirely possible and quite common to fire pistol rounds from rifles adapted for the purpose and vice versa.

Right, but if you're loading five pistol rounds into a low-capacity magazine attached to a rifle, your ability to carry out the mass shootings of Orlando or Oslo are low. It's not the long barrel or stock that we're concerned about here -- it's the energy behind the bullet and the ability to shoot a lot of them at once, without any need for that kind of bullet or the ability to shoot a lot of them at once, that is concerns regulation.

You could saw the barrel off an M-4, and that doesn't suddenly make it a pistol. Putting five pistol rounds into a magazine on an M-4 makes it no more deadly in a mass shooting context than a five-round magazine-loaded pistol.

Furthermore, why would self-defense with a rifle be unacceptable when compared to the same activity with a pistol?

Basis of need. Nobody needs rifle rounds to defend their home. In most contexts they do nothing but raise the likelihood you shoot through a wall and kill someone else in the home you're hoping to defend. You're better off with a pistol or shotgun.

This concern doesn't really involve most hunting rifles because their rate of fire is low and someone is unlikely to use one in a high-fatality shooting.