SO essentially you could make a lower (100%), decide you don't like it after you test fire and then sell it.
I read somewhere that ATF has a suggestion (not a rule or a law) that you should possess the firearm for a year before transferring it. The way it was worded was that it was not a hard and fast rule but rather a strong suggestion so that it does not appear you are making for sale by merely test firing and selling. I am sorry I do not have a cite for this mostly because I dont recall exactly where or the context in which I read it outside of homebrew firearms.
To illustrate this lets look at a California thing that recently happened. The logic of this applies to the feds. Police are exempt from certain firearms purchase laws. They bought guns ordinary folk could not get and in most cases immediately transferred them to non-police (which by itself was legal). They were charged with being an unlicensed firearms dealer and some other related things. Everything they did in a single transaction was legal but because of quantity and regularity it became illegal.
ATF similarly should never view your actions in the same light, that you are making receivers (past 80%) for the purposes of transferring them. If you make what they consider a firearm and do a private party transfer regularly they can have their jimmies rustled and retaliate. Because of "constructive possession" they can make up a whole slew of other things (shoe strings, copper pot scrubbers, etc have been ruled to be "machine guns" under the NFA). It is usually best to not rustle their jimmies.
The law itself (minus any ATF creative flair): http://www.law.cornell.edu/uscode/text/18/921
18 USC 921 (a)(10) The term “manufacturer” means any person engaged in the business of manufacturing firearms or ammunition for purposes of sale or distribution; and the term “licensed manufacturer” means any such person licensed under the provisions of this chapter.
Even if no money changes hands they can still, and probably quite successfully, argue that you were acting "for the purpose of distribution". They would have a hard time proving, without statements from anyone involved, that an occasional one-off transfer would qualify. However if you are doing this in any volume or frequency it would not be hard to convince 12 retired postal workers who comprise your federal jury that you are guilty.
There is also this which doing custom work for friends may come into play. Again its a quantity thing, will they care if you did it for a friend or two? probably not but if you do it for their friends and their friends and have a regular thing going then maybe.
(11) The term “dealer” means
(B) any person engaged in the business of repairing firearms or of making or fitting special barrels, stocks, or trigger mechanisms to firearms, or
The more time that elapses between creation and transfer the harder it is for them to argue this. The less likely it is that you would have to buy a new front door from it being kicked in at 4 in the morning.
Proving creation date is hard without receipts (either from you or the vendors you sourced parts from) to show when things were ordered or manufactured. Barrels often have serial numbers on them and they can go to the manufacturer and ask when it was produced setting the oldest it could possibly be. They can also find someone who knows you and squeeze them to introduce you to them and then try to get you to make at least 1 just for them and if you agree you are done.
I guess what I am saying is if you are planning on "deciding you didnt like it after the test fire and sell it" then goto ffl123.com and get everything you need to know about being a "kitchen table ffl" and do it right. If not dont regularly transfer stuff quickly after manufacture, let it sit for a while if you have to. Dont give them a reason