This is a post from the gunsmiths forum
http://thegunsmiths.com/phpBB2/ archives which might help answer the question.
A few years ago I had a firearms manufactures license and The local police wanted me to work on their M-16's, I contacted ATF to see what had to be done and I was told to apply for a manufactures license of nfa weapons. the law is a little different now but here is a copy of the rule from atf's site,
http://www.atf.treas.gov/firearms/faq/nobackfaq2.htm
(M11) How does a person qualify to import, manufacture, or deal in NFA firearms?
The person must be licensed under the GCA and pay the required special (occupational) tax imposed by the NFA. In addition, an importer (except importers of sporting shotguns and shotgun ammunition) must also be registered with ATF under the Arms Export Control Act of 1976.
After becoming licensed under the GCA, he or she must file ATF Form 5630.7 with the appropriate tax payment in the entire amount with ATF. [26 U. S. C. 5801, 18 U. S. C. 923, 27 CFR 47.31, 178.41, 179.34 and 179.193]
(M12) When must firearms special (occupational) taxes be paid, how much are the taxes, and how are they paid?
On first engaging in business and thereafter on or before the first day of July, these taxes must be paid in full. The current taxes are set out in the following table. Taxes are paid in the manner discussed in Question M11, above.
SPECIAL (OCCUPATIONAL) TAX RATES UNDER THE NFA CLASS OF ANNUAL TAXPAYER FEE
1 Importer of Firearms (Including "Any Other Weapons") $1000.00
2 Manufacturer of Firearms (Including "Any Other Weapons") $1000.00
3 Dealer of Firearms (Including "Any Other Weapons") $ 500.00
4 Importer of Firearms (Including "Any Other Weapons") $ 500.00 REDUCED*
5 Manufacturer of Firearms (Including "Any Other Weapons") $ 500.00 REDUCED*
* REDUCED = Rates which apply to certain taxpayers whose total gross receipts in the last taxable year are less than $500,000. )
With the 500.00 license I was able to build 30 different types of full auto rifles and pistols and didn't have to pay tax on them. but when I gave up my licenses I had to destroy them that was the only bitch.
This covers the feds but their may be state requirements that also have to be met. My work on the different guns was for R&D work I was trying to design my own selective fire weapon and improve existing systems.