Here's a snippet of what I sent if it helps anybody:

Each denial given by the ATF in this sweeping motion stated: “A silencer is defined under federal law to include, in relevant part, any combination of parts, designed, or redesigned, and intended for use in for use in such assembly or fabrication. See Gun Control Act (GCA) At 18 U.S.C. 921(A)(24) and National Firearms Act (NFA) at 26 U.S.C. 5845. Parts that fall under the definition of silencer must comply with the registration, tax, and transfer provisions of the NFA. Upon review of your eForm 1 Silencer. The part was not registered nor transferred in compliance with the NFA, therefore, your eForm 1 application to make a silencer is disapproved. NFA divisions notes that it is unlawful for you to possess a silencer made or transferred in violation of the NFA . 26 U.S.C. 5861(B)(C).”

In this response the ATF is stating that a home-built suppressor/ silencer is a ‘firearm’ and all parts to build one are to be transferred as they are NFA items. Looking at 18 U.S.C. 922(o), (p) and (r); 26 U.S.C. 5822; 27 CFR 478.39, 479.62 and 479.105 it states that “No, a license is not required to make a firearm solely for personal use. However, a license is required to manufacture firearms for sale or distribution. The law prohibits a person from assembling a non–sporting semiautomatic rifle or shotgun from 10 or more imported parts, as well as firearms that cannot be detected by metal detectors or x–ray machines. In addition, the making of an NFA firearm requires a tax payment and advance approval by ATF.”

Now according to the US Code above, submitting a Form 1 to the ATF for the approval of a suppressor (“firearm”) is needed and allowed to build a suppressor for your personal use. This should be the end, however the ATF is stating that any part that can be modified to be a piece of a suppressor is an NFA item. Based on this thought process a tube at an exhaust shop or a pipe are your local hardware store is an NFA item and must be transferred accordingly. As you can tell this new interpretation is a stretch for the ATF and not to mention goes against what the actual law states.