It is my understanding that using a pistol receiver on Mechtech CCU does not create an SBR as long as the barrel is over 16 inches long. The overall length only matters when the firearm is made from a rifle.
If the weapon has a stock, it
MUST be >26" OAL and >16" barrel,
PERIOD. Otherwise, it is SBR.
With
NO shoulder stock, it is a pistol. If >26" OAL, it
may be considered an "other firearm" for the purpose of installing a vertical forward grip, which is not lawful to do on a pistol. Bear in mind, though, that
concealing an other firearm instantly turns it into an NFA regulated AOW.
--Rifle with <16 inch barrel is NFA
--Weapon made from a rifle less than 26 inches long is NFA
It's not either/or on the 26" OAL and 16" barrel. If the weapon has a stock, to remain title I, it must be BOTH >26" OAL and >16" barrel. It does not matter if it began life as a rifle or not.
The difference lies in that a rifle may
never be made into a pistol or other firearm without the stamp, while a pistol can be made into an other firearm or rifle legally, and converted back and forth at will.
--Weapon made from something other than a rifle (e.g. 1911 receiver, Glock receiver) is NOT made from a rifle, and therefore NOT NFA if the barrel is 16 inches or longer
AND has an OAL of >26" if it has a shoulder stock.
I am not concerned that putting my Glock receiver on my Mechtech CCU with wire-collapsible stock in an SBR (OAL < 26").
So long as it sports a >16" tube and measures >26" OAL with the stock unfolded, you're GTG; OAL with folding or collapsible stocks is measured with the stock in the unfolded or extended position.
Just to be 100% clear for everyone, one more time:
a title I rifle must have a 16" barrel
and 26" OAL
a title I shotgun must have an 18" barrel (or barrels)
and 26" OAL
an other firearm must be 26" OAL and have
NO shoulder stock
A handgun is, federally, any firearm with no shoulder stock, regardless of barrel or OAL.
The distinction of "other firearm", once again, only comes into play for handguns where attaching a vertical forward grip is concerned. Handguns, however, cannot be smoothbore and remain title I, so the "other firearm" classificationis more significant, as PGO shotguns may have a barrel of <18" so long as they are 26" OAL. However, they
must have started life as an "other firearm", whether a virgin receiver or something like
this; you cannot hack the stock on your 870 and cut the barrels down to 14", as that would be an SBS.