You must be licensed as a "Manufacturer" FFL, only IF (a) weapon in question is a National Firearms Act weapon, or (b) it will "enter the stream of commerce". The BATFE regulates firearms that are transferred to others. If you want to get a REALLY sharp pocket knife, and a chunk of steel, as far a Federal law in the US, you are welcome to whittle yourself a 1911A1. FOR yourself. Or get a chunk of plastic, and make yourself a Glock.
Now, if you want to make them to SELL- or to give to another- get the license, and an accountant- you will have Federal Excise taxes to pay. Unpaid taxes put Ithaca out of business.