What constitutes an NFA "firearm made from a rifle"?

[hk940]

A while back I read some where the ATF had limited the length of a buffer tube on a pistol, if a rifle buffer tube was used on a pistol it "could be shouldered". Has any one heard of this?

 

[dogtown tom]

A while back I read some where the ATF had limited the length of a buffer tube on a pistol, if a rifle buffer tube was used on a pistol it "could be shouldered". Has any one heard of this?

No such ruling.
An AR pistol can have any length buffer tube.
The buffer tube length has nothing to do with anything regarding "could be shouldered".