There is a long winded explanation for this, but that particular example would be considered an SBR based on the new definition of a "rifle". Two of the new definitions are;
(v) The manufacturer’s direct and indirect marketing and promotional materials indicating the intended use of the weapon; and
(vi) Information demonstrating the likely use of the weapon in the general community.
The ATF is going to tell you there are ads from companies, or photos they have found, or that people have submitted to them, etc... that show a person using that exact rifle tube as a shouldering device. And then they are going to show you people who posted on the internet forums or in videos on youtube of people firing that exact pistol tube from the shoulder.
Oh, and this part of the definition;
(iv) Whether the surface area that allows the weapon to be fired from the shoulder is created by a buffer tube, receiver extension, or any other accessory, component, or other rearward attachment that is necessary for the cycle of operations;
The foam increases the surface area.
