Can someone explain to me the difference between the different Bolt Carrier groups?

M-16 "standard"............AR-15 "Semi"..............SP1 "Half Circle"

The auto is heavier.  If you can see a good pic of both somewhere, look at the rear of the carrier.  you'll notice there is more steel on the autos

beat me to it

and in your picture, the AR-15 semi, is not the original carrier. 1st ones had a square cut, and you could put an adaptator for a DIAS to work... on your picture the cut is round as they made them afterward.

See the difference on my M4 SDI carrier and my Sporter , that is what I am talking about.

If I understand correctly––an M16 BCG will work in a semi AR15 but not the other way around. Correct?

correct

Ignorant question:
Any legal concerns with putting an auto bolt in a non-NFA AR15?

None at all.

Yes, a semi BCG won't trip the m16 auto sear properly.






This is one of those subjects which has been repeatedly beaten to death.



http://www.ar15.com/forums/topic.html?b=6&f=17&t=262114



 

Gotcha.  Thanks!

OK Then I can get any BCG available for an ar-15 build.  The Auto BCGS are going to be tougher... as well as a M16 BCG

Does anyone know what the weights are of the respective carriers?

I understand that you can you can use a F/A BCG in a semi AR 15, but not vice versa as stated previously. Also, that the F/A BCG is heavier.  
Let’s put the legal issues aside (discussed here as well as other threads). I have just 3 questions, but I’m sure more will arise.

1.) Would there be any benefit to using the F/A over the S/A carrier in a semi –AR15?  Pros & Cons? (same price – so that’s not an issue)
2.) Concerning LMT – There’s a $156 difference between the Enhanced BCG vs. Standard BCG, worth the difference? (see LMT note below)
3.) LMT vs. BCM ($130/$129) I understand that the BCM is MPI tested, and I was under the understanding that the LMT was as well, so what would be the choice between the 2?

LMT notes on their site that the Enhanced BCG, “Includes both our enhanced carrier which incorporate a longer dwell time allowing barrel pressure to be greatly reduced and promoting easier extraction, and our enhanced bolt which has a more resilient coating and stronger heat treat, as well as the cartridge case being fully supported by the bolt face. For usage with full-automatic rifles.”

BCG
1.) LMT 5.56 Enhanced F/A or S/A BCG - $285
2.) LMT 5.56 Standard F/A or S/A BCG w/Enhanced Bolt assembly - $226
3.) LMT 5.56 Standard F/A or S/A BCG - $129
4.) BCM F/A BCG (MPI) - $130

BC only
1.) LMT 5.56 Enhanced F/A or S/A BC - $120
2.) LMT 5.56 Standard F/A or S/A BC - $88.80
3.) BCM F/A Bolt carrier $79.95

Bolt assembly
1.) LMT 5.56 Enhanced Bolt assembly - $155
2.) LMT 5.56 Standard Bolt assembly -$66
3.) BCM Bolt assembly (MPI) -$57.95