When does working on guns require an FFL?

[vaglocker]

If I was simply installing aftermarket sights on handguns for a fee while the owner was present would this be considered official "gunsmithing" for which I would need an FFL? If not when does that line get crossed?

[SW-Shooter]

I believed the ATF made a change sometime ago.:confused:

ETA: I think it has something to do with overnight storage

[LOKNLOD]

If they only brought the slide, would it make a difference?

[Steve_Morrison]

Slide only not a problem.
If you are working on serial numbered receivers/firearms, and you are doing it for profit, you need an FFL to be in the good graces of the ATF.