I used to know the answer to this and forgot. Digging through the ATF web is a pain.

Do you need to register with ATF to build a rifle if you have no intent of selling it or giving it to someone else. I am getting ready to build an experimental 50 BMG falling block action and would rather not have to jump through hoops. I know if you look through the ATF production reports each year, there are a bunch of people listed as one, two or a few rifle producers. I think this is for resale though. If it is for personal use, do you need to register as a manufacturer?

As far as I can determine, an individual can build a firearm for himself and is free of federal regs as long as it complies with NFA rules, that is, it's not a machine gun or a sawed-off shotgun.

Federal Firearms Regulations Reference Guide-2000 Edition, pg. 135, para.A7:
"With certain exceptions a firearm may be made by a nonlicensee provided it is not for sale and the maker is not prohibited from possessing firearms. However a person is prohibited from making a semiautomatic assault weapon or assembling a nonsporting semiautomatic rifle or nonsporting shotgun from imported parts. In addition the making of an NFA weapon requires a tax payment and approval by ATF...." Some other stuff about machine guns but looks OK.

Your best bet is to play it safe and talk to someone directly at the BATF. Get their name and extension if they tell you that it's okay. That way you'll at least be covered if they were wrong. This is a much better approach than assumption, for sure. At least at this point, you'll probably just have to buy the manufacturers license and serialize it, rather than losing the damn thing in confiscation.

ArtS, my quote above is from "Department of the Treasury, BATF, Federal Firearms Regulations Reference Guide-2000 Edition", pg. 135, para.A7 in the Questions and Answers Section. It's published by BATF and sent out with updates to all FFL's among others, I have an 03FFL which is why they send it to me.

Hardly what I would consider an assumption. I don't think they can make it any clearer that, except for the mentioned items, it's ok to build a firearm not for resale.