none,
One who is neither a FFL nor SOT, but only owns
weapons regulated under the National Firearms Act, the law seems
clear, but practice is a little murky. ATF may only compel you
to show an agent upon request the registration paperwork, that
is the Form 1, 2, 3, 4, 5 or whatever else might have been used
to register the weapon. See 26 USC sec. 5841(e). They do not
have any right to compel you to show them the weapon. However
they apparently (I have no first hand knowledge) take the
position that they can compel one to show ATF the weapon upon
request, even if the owner has no FFL. As always the Fourth
amendment applies, and ATF may not enter your home or other place
of storage of the NFA weapon, nor seize the weapon, without a
warrant, or without falling under an exception the Supreme Court
has created to the operation of the Fourth amendment. They
should also need a warrant to compel a non-FFL holder to show
them the weapon, and I would insist upon that, myself.
I have owned Machineguns, Sub-Machineguns, Suppressors and Short Barreled shotguns for 20 years now and have never heard from anyone in the ATF other than when I have contacted them.