The amount of dis-information on home built guns is unbelievalbe, and people really think they know that "it's illegal". I know I've been admonished for being un civil on this subject, but it annoys me when I see people pass on as fact that which they DO NOT know. I know that gun laws are complex and we don't all know them all, but there seems to be a disposition toward saying "you'll go to prison" when the correct answere is "I don't know"
There is no law (at least at the federal level, local laws may and do vary) saying a private party can't make a firearm for their own use. In short, the gun has to be legal and the owner has to be elligable to own guns. The limitations on the gun and owner are the same as a store bought weapon (barrel length, no full auto, all the regular restrictions). You can make a rifle, pistol or shotgun for yourself. If you want to sell them or make them for someone else you need licensing.
You'd do well to research the actual laws. BATFE has a Q&A section on their sight that's refreshingly helpful as government web info goes. You may also want to check out weaponsguild. That's a sight dedicated to people who build weapons for fun. It's helpful on many levels.