You used a key word.
"Loophole"
U don't want to base decisions on a "loophole"
Also yes, start a legit business that sells to law enforcement and any type of LEO that can/will give you the law letter you need. Which will probably be NO one since you are NO one in the firearms business.
The guys you see building, playing, showing on YouTube with the full auto stuff are usually people who have either
1.paid to acquire RDIAS, or pre-Sample and paid the top dollar.
2. Is a legit manufacturer that is well known in the business or has the proper connections and contacts to receive the required law letters to produce post samples.
3. Borrowed from someone from 1 & 2 to use in YouTube videos, review etc.
Have a full auto gun might seem "cool" but that fun switch costs ALOT of money to maintain and I don't mean just the ammo to shoot it.
Your best option is to buy a RDIAS or a MG that's for sale that's registered as NFA