Ok it's a gray area,,Over on the gunsmiths fourm it widely stated that you don't "need" to mark a "homebuilt" firearm if. and it's an IF, it's for your own personal use and NOT for sale..
However if being built for sale then yes you need a manufacturer's license and it has to be marked with a serial number to be registered...
Now according to the ATF....
All firearms
manufactured, imported, or made
on and after January 30, 2002,
the engraving, casting, or stamping
(impressing) of the serial
number must be to a minimum
depth of .003 inch and in a print
size no smaller than 1/16 inch.
And....it states that you need to fill out an application and pay the tax on all firearms..
( fourm found here )
http://www.atf.gov/forms/download/atf-f-5320-1.pdf
Now comes the "gray area". if your not building your Borchardt with the intent of selling it, would you be considered a manufacturer? and if not, would it need to be marked?
Probally the best thing to do would be to contact the ATF and ask...(800) 800-3855