Spikes Tactical engraved one of my lowers many years ago where it was just barely scratched into the metal. At that time there was most certainly a .003" minimum engraving depth and there were guidelines that stated it had to be at least .003 inch deep from the surrounding flat surface, not necessarily from the top of the edge of the markings, such as if it were stamped in and the edges were elevated. That's being pretty specific and I don't know if the NFA still states that or not in the NFA handbook. Whether or not it will ever likely be inspected, I am not one to poke the bear so I had it re-engraved by someone who actually followed the then NFA guidelines. Needed or not, I sleep better knowing that if push ever came to shove, that is one less thing I have to worry about.