It is correct a 33 round clip is the max the factory offered. A clip is a pretty simple device so even building one from scratch isn't that hard....it may jam a lot though if you don't get the tolerances tight enough.
18 bullets when is quite a bit, assuming you are going to shoot a 3 round burst, that gives you equal capacity to a 6 round revolver.
Fully automatic weapons are usually never for accuracy, even long distance....you are usually trying to 'mow' something down, otherwise you pull out the trusty 20' rifle with 1000x zoom and become a one hit wonder

.
Now getting a Class 3 license is not impossible....however most will have to prove they want to become dealers in them or have some great need (I am sure there have been celebrities denied this license and not because they were felons

).
The problem is even a cheap fully auto type weapon is going to be a few thousand and may not even work (a lot of military weapons are broken or very worn and would need a restoration to be more useful than a collectible). Other things many machine type weapons need are extra barrels and the like if you plan on starting a war

, these are also hard to come buy. However anyone with a little gun knowledge and a metal working shop could probably do a whole gun....the private gun workshop is where many of the world's weapons were concieved when the potato bazooka got boring

or that damn bear / fox kept coming back.