This will be my 1st build and I am a newb.  However I've spent hrs reading here, learning a lot!

What is the difference in operation between the M16 and the AR15 BCG's? If I'm building an AR 15, will the M16 BCG work like a regular AR-15 with all AR parts?

BTW, I live in Cal., is this a problem buying the M16 BCG?

Full Auto BCG will be fine, not needed but will work. Not sure about the cali thing, probably illegal. Just buy a reputable brand. get the full auto bcg from bcm and call it a day. If you can find one.
 

M16 BCG is the way to go imo. BCM would be a great choice.

I've had very good performance though with my cheaper Core15 M16 BCG that I purchased last fall.

In Cali.. The m16 BCG is 100% legal.. Buy from a reputable company.. DD, BCM, fail zero.. Etc

The difference is in the carrier. An M16/auto carrier has less material milled out at the rear/bottom of the carrier so it can hit the auto sear. It also makes it a touch heavier.

Either will work just fine. On a carbine length gas system you might appreciate the extra weight of a M16 carrier. On a rifle length gas system you might appreciate lower weight of  the AR-15 carrier. Personally I would recommend whatever you can find cheapest from a reputable brand right now.

If you want to see a list of who has what in stock and prices I am tracking about 400 BCG's  here on a webpage I made.

Given the use these days I see no reason for semi carriers to still be made.  The industry needs to standardize on the F/A carrier.  IMHO.

That I agree with.

M/16 carrier is the way to go.

These are my M16 full auto BCG that shows properly staked gas keys, the one on the left is a Colt the rest are older LMT (non MIM gas keys).




+1  It is not unlawful to utilize a M16 machinegun bolt carrier in a semiautomatic AR15 type rifle (unless you have a state or local law that specifically states it).

Major gun manufacturers that sells California compliant AR the good ones sells with M16 BCG (for example Colt and others).

Desirable features to look for are mil-spec manufacturing process such as shot peened bolt, high pressure tested (HPT) and magnetic particle inspected (MPI) bolt, chrome lined gas key and carrier, and properly staked gas key.

Some of the recommended brands that have these noted desirable features:
Colt
BCM
Daniel Defense
Rainier Arms ThunderBolt
AR Performance (9310 steel alloy) SuperBolt
Spike's Tactical

What advantage is the "Enhanced Nickel Boron " or is it just fancy salesmanship?

http://www.joeboboutfitters.com/JoeBobOutfitters_Enhanced_Nickel_Boron_Complete_Bo_p/jb-enhancedbcg-nib.htm

Or would I be better getting a Youngs BCG like this one?

http://www.kearms.com/bolt-carrier-m16

The NiB (Nickle Boron) finish is harder, wears less and has more natural lubricity than a phosphate finish. There are other upgraded treatments on there like QPQ, NP3 and Ion/black Nitriding to name the more common ones.  

There are pro's and con's to all of them and it really depends on what you want out of your rifle or how you use it. If you expect to be able to add oil to rifle after firing up to 10 magazines and you don't expect to ever use your rifle so much you will have to replace the barrel at any point, then you may not find any of the upgraded treatments worth the money.  But if you want a hard running duty gun, the extra cost is worth it (to me it is anyway).

I am in the process of testing most of the treatments myself and it will be while before I am done. But my initial impressions are NiB wears well but is not slick without oil. QPQ wears faster but is smoother without oil and NP3 is the slickest of all, but I have no idea how well it wears yet. I also have not used any Ion NItrided carriers yet, but the treatment has potential for high wear resistance. They customize the treatment to the base metal and I am told, if they do it right it can give more then 5x the wear life of the nearest competitors. Their big pitch is hardness though, rather than natural lubricity, though they say that is improved also.