I have not had the chance to adsorb all the information here but it's the first time I have found a place that directly discusses tax payments on home builds if you sell one...
http://www.ttb.gov/fet/faet_gunsmiths.htm
3. If my customer is considered the manufacturer, when is he or she liable for FAET?
The customer is liable for FAET if:
• He or she sells the firearm before using it; or
• He or she uses the firearm in the operation of his or her business.
The customer is not liable for FAET if:
• He or she uses the firearm for personal use after delivery from the gunsmith.
Also see:
http://www.ttb.gov/fet/faet_manufacturers.htm
4. If I manufacture an article only for my personal use, am I still liable for FAET?
If you manufacture a firearm for your personal use, you are not liable for FAET. (Note: this exemption does not apply to partnerships and corporations)