You could contact the BATFE directly. They don't have many "gray" areas.
That sounds good in theory, but usually ends in tears. ATF thrives on gray areas. The ATF exists to enforce random, arbitrary rules that they make up as they go. If you ask five different ATF agents a question, you will get five different answers, and five of them will be wrong. Most won't even try to answer, because they know they don't know, because it's not possible to know. Even if you do accidently get a 'correct' answer, they'll probably change their interpretation of the law soon, and render you a felon anyway. ATF rules are similar to the Youtube terms of service. They will not tell you what they are, but they will tell you when you have broken them. Then, they'll shoot your dog, take all of your stuff, and put you in jail. Not sure if Youtube does that last part.
For the most recent high profile example of this, read about the Tommybuilt T36. They told him they decided it is a machine gun, even though they have allowed them to be sold for several years. They won't say how they tested it, they won't give him anything in writing, and they won't tell him what he needs to do to fix it. They are aware that there are other guns still for sale in the US that are exactly the same as the T36, but they only declared the T36 a machine gun, and not the other. There are many other examples just like this one, if you care to look.
As another example of how the ATF doesn't like gray areas. Let's talk about AOWs.
You can put a vertical grip on an AOW, right? ...but when you read the AOW law (26 U.S.C. § 5845(E)), the last sentence says 'Such term shall not include a pistol or a revolver having a rifled bore, or rifled bores, or weapons designed, made, or intended to be fired from the shoulder and not capable of firing fixed ammunition.'
The important bit being 'such term shall not include a pistol or revolver having a rifled bore'. Hmmmmm. Your SP5K is a pistol, right? It has a rifled bore, right? The law says it can't be an AOW. IT CAN'T BE AN AOW....yet it would seem, it can. Can't it?
26 U.S.C. § 5845(E) uses the word 'pistol', but there are different definitions for 'pistol' and 'handgun'. Turns out, the SP5 isn't a pistol, it's a handgun.
US Code Title 18 Part 1 Chapter 44 Section 921
(a) As used in this chapter—
(29) The term “handgun” means—
(A) a firearm which has a short stock and is designed to be held and fired by the use of a single hand; and
(B) any combination of parts from which a firearm described in subparagraph (A) can be assembled.
Code of Federal Regulations Title 27 Chapter II Subchapter B Part 478 Subpart B Section 478.11
When used in this part and in forms prescribed under this part, where not otherwise distinctly expressed or manifestly incompatible with the intent thereof, terms shall have the meanings ascribed in this section. Words in the plural form shall include the singular, and vice versa, and words importing the masculine gender shall include the feminine. The terms “includes” and “including” do not exclude other things not enumerated which are in the same general class or are otherwise within the scope thereof.
Handgun.
(a) Any firearm which has a short stock and is designed to be held and fired by the use of a single hand; and
(b) Any combination of parts from which a firearm described in paragraph (a) can be assembled.
Pistol.
A weapon originally designed, made, and intended to fire a projectile (bullet) from one or more barrels when held in one hand, and having (a) a chamber(s) as an integral part(s) of, or permanently aligned with, the bore(s); and (b) a short stock designed to be gripped by one hand and at an angle to and extending below the line of the bore(s).
So, you want your 'handgun' to be an AOW. Let's forget the pistol/handgun definition discrepancy for a moment. Title 27 Chapter II Subchapter B Part 478 Subpart B Section 478.11 says a handgun is 'any firearm [snip] designed to be held and fired by the use of a single hand' ...but the SP5 has a 'handguard'. How can the SP5 be sold as a 'Handgun' when it clearly does not meet the definition? Since handguards are obviously allowed on handguns, since both the SP5 and SP5K come from the factory with handguards, where does it specify that a vertical handguard is bad, but a horizontal handguard is OK? Where does it say if you pronate your wrist when you two-hand your handgun, you are good to go? Here is a hint: It doesn't.
...But 26 U.S.C. § 5845(E) says 'pistol', and the SP5 is a 'handgun', so it doesn't apply anyway. Right?
Yeah, ATF hates gray areas.
Here is how to NFA within the current legal framework:
-Read all of the existing laws, rulings and letters issued by the ATF that you can find. Note that many of the letters and opinions will directly contradict each other.
-Read gun forums, talk to other NFA owners and listen to their shared experiences
-Attempt, in your mind, to establish ATF enforcement trends and objectives
-Decide what your threshold for risk is
-Make a decision, and proceed.