First I know next to nothing about class 3 stuff and the pertaining laws.
If a person were to find a full m-16 fire control group, all the internals, does it need a stamp? If you would own this and have ar lowers, without the 3rd hole would it be considered " constructive intent" and illegal?

Having both the FA BCG, and any lower could be a bad idea

Having a full M16 FCG with an AR lower is a great way to experience sodomy in a federal penitentiary.

I've also been wondering the same thing OP, i see FA lpk's with sears for sale everywhere, hell you can even buy them on Brownells.

So they are legal to buy, but if you have a lower receiver they can rape you for constructive intent?

Parts are parts, and the sear won't fit an unmodified lower anyway.

HOWEVER. In an old advisory opinion, BATF cautioned that if their technical experts can get your rifle to fire more than one shot with a single trigger pull, ever, they consider it to be a machinegun. They further caution that if you install a full set of M16 FCG parts in your lower, that their technical experts can indeed get it to fire more than once per trigger pull, so they advise against doing so.

Basically, with an M16 selector, bolt carrier and disconnector installed, the hammer will follow the bolt carrier into battery with the selector in the "AUTO" position, and with special ammo using light primers, it can be induced to slam-fire. Although this can happen with semi FCG parts, too, apparently BATF considers the use of M16 FCG components to make that particular malfunction intentional, rather than the case of mechanical failure it would be considered if it happened using semi FCG components.

It's been a long, long time since the opinion was issued. The only conviction I ever heard of related to it was Olafsson.

It would be considered a machine gun as per definition.

26 U.S. Code § 5845 - Definitions

(b) Machinegun
The term “machinegun” means any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger. The term shall also include the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun, and any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of a person.

Then how in the hell do all these retailers get away with selling FA parts kit/sears? 99% of folks who buy them definitely have a lower receiver, so essentially there's thousands of people guilty of a felony?

No law prohibits sale. Hence the disclaimer you usually see "All NFA laws apply".

They are REPLACEMENT PARTS for LEGAL machine guns, either transferables, law enforcement rifles, or "Post-'86" Dealer Samples.

The way I understand it is;

You can have all the fa fcg and bcg as long as you have no lowers at all.

You can have a fa bcg in your lower as long as you have fa fcg parts.

I do not think you can have a fa fcg in your posession with a semi bcg and a lower.

I wouldn't even have the fcg without the sear if you have any lower.

You could buy the fcg after selling your ar's if you're a crazy prepper. In the doomsday shtf worl you could fairly easily find an ar, drill a hole and have a machine gun. There would be no government left anyways then.

I'll just keep my ar and stay legal myself.

They aren't doing anything illegal.  It is the same with "solvent trap" parts, ar "pistol" barrels, drive through convenience stores selling alcohol. As long as they have no reason to suspect you of breaking the law they aren't liable. If they think you are going to use the stuff in an illegal manner and still sell to you, they can be held liable.  

Call brownells and order a fcg, then ask for the specs on where to drill the sear hole.  I bet your order gets cancelled and you might get a knock on your door!

if memory serves me well olafsson was also a major league dum dum and had done some extensive home gunsmithing on his trigger kit.

The HK MR556 and MR762 both come with full auto fire control groups, and a semi auto safety.

This.

A full auto trigger/disconnector plus a full auto selector will get hammer follow and can discharge more than 1 round per trigger pull.

M16 bcg's are common in AR15's, no issue.
M16 hammers are often used in 9mm AR15's, no issue.
Put the 3 listed parts together, big issue!

ETA- The short and skinny of my point is, don't ever have an M16 disconnector and you are fine. Hell, I saw a guy at the range running a spikes tactical with the pictographs and an M16 selector, when I asked why, he replied "it looks cool". TETO.

edit

there are thousands of transferable, pre-may and post-may samples out there which need Full Auto parts kits. Most companies make it very clear you are responsible for following all NFA laws.

do some people buy the lower kits and install them into title one semi-autos? sure, but I wouldn't recommend it as it could land you in prison.

As a civilian if you want to shoot full auto your choices are a transferable, bump stock or binary trigger