why engrave at all, when I called the ATF and asked if it wasn't necessary they said only if I plan to sell it in the future was the engraving necessary, I have had my SBR for 4 years and had an ATF agent friend look it at it and he said the same thing as long as I have the stamp and never sell it I am good
This is true of title 1 firearms only. If you build a rifle from scratch you don't legally have to even put a serial number on it unless you sell or otherwise transfer it.
On the other hand this is not true of title 2 / NFA firearms. For them the regs very clearly state that you (the maker or manufacturer) must mark it with Name, City, State, Cal, Serial and model. The serial number must be on the receiver, the other info can be on the frame, receiver or barrel but cannot be put in an area that is obstructed or covered in normal use. It must be conspicuous so you cannot put it under handguards or grips etc.
As for whether you can engrave prior to the form 1 being filed or approved it is as noted previously. You can engrave whatever you want at any time on a title 1 firearm. Once it is NFA as in short barreled rifle, suppressor, MG, DD etc than the marking regs change and you cannot alter or move them. That means if you decide to change your HK 94 pistol into an MP 5 PDW you need to alter the markings prior to registering. Weld up the HK 94 marking and re-engrave the MP5 marking before you send in the paperwork.
Details, details, details.....
I for one wouldn't engrave the lower at all. Its unnecessary and just clutters things up. Engrave the barrel if there is enough sticking out or engrave the upper. Read the regs and you will see its legal and proper. All you need to add is the name, city and state in most cases since the cal is either on the lower or upper or barrel already. The serial number must be on the receiver already and you use that for the form 1 (read the form 1 instructions). That means with a normal sized name, city and state you might use up 3/4" of space lengthwise on your barrel or upper. If you wear out the barrel you can do another. If you decide you don't want it to be an SBR anymore you don't have to explain why your info is on the lower. Barrels are cheap and so are uppers.....why screw up the good part?
Just my opinions of course based on many years of dealing with ATF and the NFA.
Frank