Which extended buffer tube for SBA3?

[Lastrites]

Yet another gray from our friends at the atf, the latest info stated using a standard tube and said that the tube would have to be fully functioning as in prohibiting extended tubes such as the KAK Supertube which has solid core of at least an inch at the rear of the tube that essentially does nothing but increase the overall length of the weapon. Nothing was mentioned and shouldn't be prohibited using a Vltor A5 or a regular rifle extension as they are fully functioning so those should be able to be utililzed. The other end of the contention is the previously stated LOP measurement which was not to exceed 13.5" this may cause if using one of the longer tubes. I believe I recall seeing a Vltor tubed pistol brace and it was just short of the 13.5" mark.