You must be engaged in the business of selling Title II, or NFA, weapons(machine guns, short barreled rifles and shotguns, silences, and so on). You must also have an appropriate FFL in this case dealer/gunsmith(I believe, I'd have to find the appropriate section). The ffl will initially set you back at least $200(that's just the fee to the ATF, then you may require state and local licenses.), then then the SOT(Special Occupational Tax) 3 will run you $500 per year. Plus you just can't buy machine guns for yourself, your have to be engaged in the business of selling them. If you obtain your ffl and class 3 to just help your collection when the ATF finds out your going to have some problems.