My understanding, is the LEOs will have an eighteen, and sixteen inch stick, with a marked handle so they can pull it back out.
The drop the stick in, and if it hits the bolt while said LEO can still see unmarked measuring depth, congratulations you have an NFA violation.
Bullpups are fine because the whole stick disappears, although the other important measurement is the overall legth -- less than 26.5", I think? and again its too small of a carbine.
I don't know what the maximum overall length of a pistol is, but the same two measurments are considered for deciding it's a pistol.
That assumes they're playing by their own rules. They've also, I've heard, classified AR platform pistols as SBRs because it had a 4x scope and was chambered in "a rifle caliber" and you know, we can't have that, can we?