I know this has probably been asked before, but I couldn't find a definitive answer.

If I am concerned about accuracy in my build, using a match barrel and free float handguard, etc. What BCG is best?

Will any high-quality BCG be ok? I've heard good things about Toolcraft, Aero, and Lantac...  I've heard MANY good things about a regular Toolcraft being the best, and it's the least expensive!

I respect the opinions here, so thanks in advance.

I thought about that, but I forgot to mention in my post that I already have my barrel.

I’m using a Young Manufacturing National Match bolt carrier group in my Mk12.  They are not cheap.  

Young BCG

Honest question for the experts: does the BCG have any measurable effect on accuracy precision in an AR?

I've never even considered that as part of the equation myself.

I think it can't be much. Even the companies with "matched bolts" are just ensuring headspace.

In 20 years of building guns I have never had any accuracy difference when swapping bcgs.

That is what I've always thought too.

To OP, I've been using plain old Toolcraft for years with no failures yet in 10s of thousands of rounds, just replacing gas rings as needed. BCM has a great reputation around here too.

I say no.  As someone said before, a matched bolt ensures proper headspace.  The bolt itself, feeds, fires, extracts and ejects.  I don't see how any of that affects accuracy in a statistically significant way.  I suppose excessive headspace could affect accuracy due to sloppy lock up but then we're taking about an out of spec bolt or barrel extension, which is a moot point.

If you want to get really technical, I suppose a properly tuned BCG, gas and recoil system could affect split times and follow up shots but I've always considered the BCG to be a reliability component, not an accuracy component.

Maybe some testing is in order.

The hand loading component makes sense and I agree a greater headspace can lengthen freebore.  As for the statement that it's argued to be the greatest contributor to accuracy, I guess I don't know how significantly the items above in RED affect the round once it engages the rifling.  Is it a matter of how consistently the bolt orients the round in the chamber?  I'm genuinely curious to know more.

Negative.  Don't forget the ejector spring is forcing the cartridge forward into the shoulder of the chamber. If the headspace is on the long side, it just means that the back side of the case is not supported tightly against the bolt face. It's still being jammed forward by the ejector.

If you have a spare barrel & bolt, take a resized case or a dummy ctg and try and and lock it up with just your fingers. A new bolt (with a new spring) will take a little bit force to get it to lock.

I think the BC is the least important thing to worry about when it comes to accuracy. However if you read some of the American Gunsmithing articles by Joe Carlos, he has written that it does make a difference. He even has a mod to the back end of a BCG where he adds threaded fasteners to adjust how the back of the carrier rides in the receiver extension.

I've shot cleans at 600 yards with plain old BCG, Colt AR-15 BCGs that don't have a shrouded FP, Young's NM Chrome carriers and I think an AIM Nitrided BCG. I really like the Young's stuff, but that has more to do with the cleaning than accuracy.

If you ask John Holliger, he will tell you he just uses regular GI carriers.

When you say greatest contributor can you elaborate a bit? More important than the barrel chamber etc? I've never heard anyone claim that so if you could point me towards the info I'd appreciate it. White Oak was the main one I'd heard the check headspace with from. My understanding is that he will just check headspace if you order a bolt and barrel together.

I've seen a lot of discussions on various levels of work done but I've never seen a break down that showed an improvement after that work was done. A lot of my info has come second hand from High Power shooters. I've always felt like most of them are really good at weeding out stuff that isn't worth the time or money when it comes to accuracy. They also aren't benchrest or similar level shooters so they have different accuracy requirements.

Joe Carlos has discussed vertical play of the carrier in the receiver.  This,  (according to Carlos) can carry over to the bolt, thus canting it within the barrel extension,  degrading accuracy.

Edit,  just seen this was already touched on.

I'd go with a lightweight BCG.  Won't help with mechanical accuracy but should give a little boost to practical accuracy.

I guess I did  

You're right and thanks for the reminder.

Interesting. Would be cool to see testing results. I'm wondering if we're talking about 0.01" or 0.1" type group changes at 100 yards.

Most best is best, so I won't make fun of anyone trying to wring even the tiniest bit of precision out of a gun, but I do wonder what one could reasonably expect for a given amount of effort working on the BCG in an AR.

This post has my vote.  I have used several of their NM BCG in accuracy oriented builds and while the bolt was not matched to my barrels, these have never failed to deliver.  Of course, other things come into account other than the BC but if I were doing another build geared toward top accuracy, this is the bolt I would use again.

If anyone is interested in AG; most libraries subscribe to publications via digital services now. It saves them from having to store physical copies of publications, and brings their members a greater variety of publications.

If you have a library card, you may be able to access American Gunsmith & their archive (going back to 2012) via https://search.ebscohost.com/

Just go to the link and then click on Access through your institution and do a search.

I found that Digital Maine Library has a subscription. Once inside, select All Databases, and then click Hobbies & Craft Source -- Publications.

Once you select that database, do a search for American Gunsmith. All the articles are there in .pdf format so you get the pictures that were published, including the text. Bookmark it, so you don't have to go through the above process again.

DML seems to be free and open to the public; as I'm not a member.

Not sure about this part from that first link. Does tight upper-lower fit have any influence on mechanical precision?

He does some interesting mods later in the article to try to minimize the carrier tilting downwards in the barrel extension due to gravity. Guess he never shot his gun upside down, or maybe held out sideways while wearing a life preserver in the desert

He actually does give some % improvement numbers but doesn't talk in this article about his initial baseline or how he performed testing.

Either way interesting reads, thank you.

He ran the entire USAR shooting team upper inventory (around 200 uppers if I remember correctly) through a mechanical rest to obtain the baseline. Dude is thorough, records everything..every torque setting for every component of every upper etc. Each change of component gets tested using the same parts and torque settings as baseline than ran through mechanical rest, groups shot and recorded and then compared to baseline groups.

Everything effects precision, including magazine spring pressure differentials. You can observe changes in POI just from swapping magazines, which is one reason to single feed by hand. The reason sloppy BCG fit effects precision is because it allows the projectile to be presented to the chamber out of line and at varying angles depending on how the rifle is held. Any opportunity for variation that can be eliminated will increase precision and consistency.

Is this stuff of concern for mag dumping into a dirt pile or laying down heavy suppressive fire? No but if you are looking to squeeze the absolute most precision and consistency from the AR, these things do matter and top level competitors do utilize these methods. Think about it like an off the shelf bolt gun v.s. a blue printed and trued action bedded in a stock etc. Can you kill a deer with an off the shelf bolt gun? Sure but sub quarter MOA shooters aren't using off the shelf bolt guns. Can you get your groceries in a car with bone stock engine? Sure but top level competitive race teams are balancing and blue printing, re prioritizing oiling and cooling within the block etc. to reach the level of performance that they compete at.

I had a White Oak upper from John and it shot sub moa if that makes you feel any better.