1) "I want one o' them Class 3 guns!"
There is no such thing as a "Class 3" gun. If you want a machine gun, then what you want is a "Title 2" or "NFA" firearm. You would purchase it from a "Class 3 Special Occupational Taxpayer", which is a dealer in NFA weapons, as opposed to a "Class 2 SOT", which is a manufacturer, or a "Class 1 SOT", which is an importer. Saying "Class 3 gun" makes you sound goofy.
2) "If you own a machine gun/have a C&R FFL, the ATF can come in your house whenever they want!"
Wrong. Not without a warrant they can't. And with a warrant, they can come in your house whether you have a buzzgun/03 FFL or not, nicht wahr? If you have a commercial FFL (anything except a Type 03 Curio & Relic collector's license) they can inspect your premises any time during business hours, much like the FDA could inspect you if you were running a meat packing plant or the FCC could inspect your radio station.
If you have a collector's license, on the other hand, they can only request to inspect your books and the guns that are on them, by appointment, and you can do it at your house or go to their office. And I don't know a single cruffler who's ever been audited.
If all you've done is pay the transfer tax on an NFA weapon, the BATFEIEIO can (theoretically) ask you to produce the registration documents (your Form 4 or Form 1 or whatever has the tax stamp on it). They can't come in your house. They can't demand to see the weapon. At least not without a warrant. And if they're going to get a warrant for you, do you think they give two farts in a windstorm whether you have a Form 4?
I was talking to Oleg years back and he was pondering getting his first suppressor at the time, but said he didn't want to be on The List. "Oleg," I said, "if the ATF has a list, you're already number one with a bullet, so go ahead and fill out the Form 4. Buy a can. Live a little."