How can you get a class 3 gun license?

Answer 1

It depends on what you mean. If you mean, as an individual, a license to purchase class 3 guns, there is no such thing. Individuals can purchase class 3 items (more properly called NFA weapons). The "license" as it is commonly but incorrectly referred to, is a tax stamp. You must have a tax stamp for each item. If you own 2 machine guns and 2 suppressors, you must have 4 separate tax stamps, one for each item.

If you mean a dealer's license to sell NFA items, that is called an SOT (Special Occupational Tax), and you simply apply to the ATF, but you must be a dealer to get one of those. You can't just apply and get one because you want it.