Registered Drop-In AutoSear. If installed in an AR15 receiver along with a complete M16 fire-control part system, it allows FA function.

To answer your original question, any registered Colt factory receiver (M16s, M16A1s, M16A2s) allow you to use any current configuration –– I use my M16A1 with M4, LMG, DOE9mm, etc. uppers and it functions flawlessly. Same goes for most registered conversion AR15 full-auto receivers –– they can be configured the same as any current full-auto variant, with no receiver modification.

sounds like you're looking for the M16 lower I have in the EE right now.  

To my knowledge, the transferable factory M16 registered receivers are Colts plus 100 receivers made by Group Industries from stainless steel (instead of aluminum). The GI SS receivers are slightly heavier, of course, but also way stronger. If I wanted a bulletproof M16, I would look for one of those.

There also are rumored to be four to six transferable factory M16s registered by Bushmaster. I say "rumored" because I am not convinced they are original receivers made and anodized as FA prior to May 19, 1986.

There really is nothing wrong with conversions, except that very few were reanodized after the milling-and-drilling process, which left soft alloy exposed. An unfired or low-mileage conversion, if in-spec, could be anodized and be as strong as a factory M16.

A second issue with conversions is that many of the host receivers were either not 100% in-spec, or had occasional issues with porous metal. Remember, before 1986, most clones were not built with precision shooting or long-term wear in mind ... they usually would not pass milspec tests. So you find conversions where the host has a too-high top deck, or too-tight out-of-spec magwells, or voids in the forging that make repairs difficult or impossible. With the exception of voids, all can be repaired and brought back into spec, then the receiver re-anodized ... though repeated anodizing also has a price.

My ranking is to place factory Colts at the top, because so much of the M16 development was done by Colt, though an SS GI receiver could arguably be better for someone who values durability over originality.

Next I would rank Colt SP1 conversions –– yes, I know the front takedown pin is the wrong size, but it can be remedied by either using large-pin uppers, an offset pin adapter, or sleeving the holes so you can use milspec uppers. The upside is that the receiver quality and attention to milspec is far higher than the clone hosts.

I would rank non-Colt clone conversions next, followed by rewelds. And purely personally, I would place rewelds and remanufactured receivers at the bottom –– rewelds, because the welding process introduces weak points, and remans because I have serious reservations about their legality and thus long-term ownership.

RDIAS have their own set of problems:

––They were made by several C2s, and since there is no milspec, dimensions can be all over the place. Some are truly plug-and-play; others require tuning and fitting to each host gun and each upper. (And a seller will always swear they have no problems with theirs.)

––In addition, I have reservations about what would happen in a catastrophic failure –– I have seen KBs that scattered parts all over the range, and rarely do you recover 100% of the pieces. An RDIAS is tiny and if you find 99% of the gun but the missing 1% is the RDIAS, you just lost $15k. Yeah, that's prolly a long shot –– but so is a KB. I've found that most RDIAS owners run their guns harder than owners of transferable receivers, though, so to me it is not farfetched.

––Finally, I know of several "RDIAS" that are circulating out there on valid paperwork, but the RDIAS themselves are not original, and IMHO are just waiting to be confiscated by ATF. They were either illegally replicated, or else started life as RLLs and suddenly were transformed into RDIAS. IMHO, neither is legal.

Just some things to think about. As always, Your Mileage May Vary.

Pointman69, you're right, there's a lot to learn, and the only way is by asking questions.

You can know all the technical aspects of M16s and still be lost when you first enter the civilian market. Many members here didn't have the background info when they took a deep breath, bought what they saw ... and to this day they are happy owners. If you love machine guns –– and 99% of the folks who buy them, still love them months/years/decades later –– they are the most fun you can have with your pants on. And that applies to factory M16s, converted ARs, rewelds, RDIAS, RLLs. All are fun, and only a very very few are lemons.

I prefer factory Colts 'cause that's what I grew up with, and back then, the RDIAS had not been invented.

But RDIAS owners swear that theirs are way superior to registered receivers.

And conversion owners think owners of factory M16s are fools for paying a coupla grand extra for a pony on their magwells.

Each to his own. IMHO, the risks of buying a factory Colt are smaller than the alternatives. But yes, they are more expensive than some alternatives.

Keep reading posts here, asking questions, and when the opportunity that feels right arrives, dive in. t's sorta like getting married, except the odds of long-term happiness are much higher when buying any machine gun.