I believe if I'm reading correctly, that serial numbers have to be .003" deep and 1/16" tall, but that any of the other markings have to be .003" deep only (no height limitations). 1/16 is still nice and small though. Seems like most markings I've seen on an AR are about double that.

Found this on the PA forum posted a while ago:
http://forum.pafoa.org/nfa-class-3-t...nfa-items.html


AGENCY: Bureau of Alcohol, Tobacco and Firearms (ATF), Department of the Treasury.
ACTION: Final rule (Treasury decision).
SUMMARY: The Bureau of Alcohol, Tobacco and Firearms (ATF) is amending the regulations to prescribe minimum height and
depth requirements for identification markings placed on firearms by licensed importers and licensed manufacturers. Specifically, we
are requiring a minimum height of 1/16th inch and a minimum depth of .003 inch for serial numbers and a minimum depth of .003
inch for all other required markings. We believe that these minimum standards are necessary to ensure that firearms are properly
identified in accordance with the law. In addition, the final regulations will facilitate our ability to trace firearms used in crime.
DATES: This rule is effective January 30, 2002.