Just submitted an eForm 1 for a SBR, to attach a SIG P556 pistol upper (10in barrel) to a SIG556 lower with folding stock. I entered the Barrel as 10in and OAL as 20.5in, as that was the SIG specification for the upper. Hopefully, OAL wasn't the assembled SBR length - the only way to known that is assemble a SBR before getting a stamp (nope) or estimate by subtracting 6in from the SIG 556 (min 26.1in folder or 34.5in extended). Anyway, I was thinking the SBR requirement was barrel <16in OR OAL <26in - it didn't strike me the eForm 1 OAL might have been the full SBR length until just now.
I'll know soon...should have asked here first, in hindsight!