First thing you need to check is whether machineguns are legal in your State
If they are legal then there is nothing (outside of the CLEO refusing to sign) from keeping you from purchasing a Machinegun.
Relevant laws:
Pre 1934: Every and anything was legal
NFA (Nation Firearms Act) 1934: Created a registry and excised a $200 making tax and a $200 dollar transfer tax on the making of any SBR, SBS, MG, or Suppressor.
AOW, have a $200 dollar making tax but only a $5 dollar transfer tax.
GCA (Gun Control Act) 1968: Bans the importation of non-sporting (i.e machine-guns) from abroad. All imported machineguns after 68 are now referred to as Pre-May Dealer Samples. These guns may only be possesed by Class 3 dealers but may be kept by the dealer after allowing his SOT FFL to lapse.
FOPA (firearm Owners Protection Act) 1986:
Bans the making of MGs by civilians and bans the transfer of new MG's to civilians.
This creates a new class of MG's (Post 86 Dealer Samples) These may only be possesed by Class 3 dealers and may NOT be kept after their SOT FFL lapses.
This law also creates a new class of machineguns (Pre-May Fully Transferable) of which any domestically made machinegun made before May of 1986 may be purchased or possesed by a civilian.
You may as a civilian purchase any PreMay transferrable machingun if the following conditions are met.
1. Your state allows machinguns.
2. You are able to obtain a CLEO (Chief Law Enforcement Officers) signature for your ATF FORM 4. Most officials from major cities will not sign.
3. If the gun is already in your state owned by either a dealer or civilian the gun may be transferred directly to you via a ATF Form 4 $200 Dollar Tax.
If the gun you wish to purchase is out of state. You will have to have it transferred to a dealer in your State. If the gun is already owned by a dealer it can be transferred tax free via a ATF Form 3. It it is owned by an individual it will have to be transferred on a ATF Form 4 $200 tax and then will be transferred from your dealer to you on another FORM 4 $200 tax for a total of $400 dollar tax. However this senario will also get you out of sales tax, so depending on what you buy $400 might be cheap in comparison.
4. You can pass an FBI background check.
5. You have the cash. Since transferable machinguns are like real estate and they are not making any more of them expect to pay up big time.
Entry level gun is a MAC running at 1200 to 1500 bucks
Middle ground, M16 $6000 to $10,000 dollars
High End, M60, $16,000 to $20,000 dollars.
Exotic, Mini-gun $250,000+
To get started at the very minimum in Class 3 nowadays will require at least a $1500 and probably closer to a 2000 dollar investment for a MAC + ATF Tax + Sales TAX + a stack Mags and ammo.