Make sure you grab a pen and some paper, the National Firearms Act is one tricky legal journey.
NFA guidelines are the rules that restrict anything fun in the weapons industry. The NFA has been pretty strict when granting access to limited types of weapons. The BATF, is not the agency to mess with when trying to get you hands on some restricted weapons.
Your documentation needs to be exact when requesting permission to get certain features and you will likely require several signatures in your local area that may be difficult to get.
Careful application processes, and some tax stamp fines are required to get some of the evil features that are described on certain weapons. The BATF is always looking for ways to curtail features that are common place and will always try to protect the general populace in their decisions, even though the decisions may not be well received by the shooting industry.
The BATF has been the hot-button organization for many of 2015's ammunition bans, and that of the scandal that leaked weapons to the Mexican Cartels.
Keep good notes if your planning on taking on the challenge of applying for NFA weaponry. Pay especially close attention to NFA Trusts, and how they work within the dynamics of who can be in possession. Also pay particular attention to AR pistols as they are quickly becoming a favorite in the shooting industry.