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Old August 7, 2013, 09:48 PM   #1
SHE3PDOG
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AR Bolt Carrier Group Qustion

I'm building my first AR, and I was just wondering what makes one BCG better than another. If they are both the correct tolerances to fit within a milspec upper, is their really an advantage to having one over the other?
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Old August 7, 2013, 11:14 PM   #2
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There are quite a few differences between BCGs. Some are made out of better metal. I think 158 carpenter steel and 9310 steel are the best. At least I think they're the ones that pass mil-spec. Then there's the different coatings like Nickel boron. IMO, these come down to personal preference. Except for the chrome lining in the carrier, which is standard for most manufacturers now. Theres also semi auto and full auto carriers. The full auto ones weigh more. I think to keep down bolt speed and bolt bounce. And another thing, quality manufacturers stake the gas key, shot peen and MPI the bolt/carrier. Always pay attention to these.

Quality manufacturers like Palmetto, BCM, DD, and Spikes will have all the right features. As long as you buy from a well known company, you'll be good to go.
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Old August 7, 2013, 11:22 PM   #3
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So, for just a basic build, as long as it has a staked gas key, and has been shot peened and MPI'd, it's good to go? This was my thinking, but I wanted to be sure of it.
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Old August 8, 2013, 12:40 AM   #4
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Yes, that'd be good to go. Because if it has that stuff done to it, its probably an M16 carrier with chrome lining anyway, which is preferred.
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Old August 8, 2013, 04:10 AM   #5
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Staked gas key is what I look for in a budget BCG. The rest is just gravy if you can get it. Nothing more frustrating than having a gas key fly off in the middle of a match (happened to a shooting buddy of mine).

For what it is worth, I'm not above staking the gas key myself if it will keep the cost of a build down.

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Old August 8, 2013, 11:53 AM   #6
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Quote:
I'm building my first AR, and I was just wondering what makes one BCG better than another. If they are both the correct tolerances to fit within a milspec upper, is their really an advantage to having one over the other?
It's more than just tolerances. It's also the materials used in the construction and the testing procedures. Inferior materials make for weaker parts and improper testing procedures can let bad units slip out of the factory more easily.

While currently out of stock, I would recommend one of these.
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Old August 8, 2013, 12:42 PM   #7
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The Marines think a quality BCG is important so I'd think along those lines. You can spend $125-225+ and end up with quality or crap, even chrome plated crap so watch out.

Standard phosphated BCGs from BCM, Colt, Daniel Defense, FN, etc. are all you need. Here are some specs, yes milspecs!

Milspec Carpenter No. 158® steel
HPT Bolt (High Pressure Tested/ Proof)
MPI Bolt (Magnetic Particle Inspected)
Shot Peened Bolt
Chrome Lined Carrier (AUTO)
Chrome Lined Gas Key
Gas Key Hardened to USGI Specifications
Grade 8 Hardened Fasteners
Key Staked Per Mil-Spec
Tool Steel Extractor
Milspec Extractor Spring
Black Extractor Insert
Mil-Spec Crane O-Ring

http://www.bravocompanyusa.com/BCM-B...0auto%20mp.htm
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Old August 8, 2013, 02:01 PM   #8
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Thanks for those specs. I just want everything to work out alright the first time I assemble it. I nearly have all the parts I need to build it. I can definitely see how building custom ARs could be addictive...and ecpensive.
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Old August 8, 2013, 03:40 PM   #9
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There is a degree of nonsensical jargon thrown in lots of times. Along the lines of "baffle them with BS."
Quote:
Tool Steel Extractor
what kind of steel again? A2? D2? O1? 4000-series? lots of different steels used in tooling, some more suitable to a particular purpose than others (that's why there are more than one "tool steel").

Find one made by a reputable manufacturer and be happy.
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Old August 9, 2013, 03:59 AM   #10
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Normally if you buy a BCG from a reputable company like Colt, BCM, or DD, you are good to go. If buying from a lesser known company, make sure they have the specs listed above.
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Old August 9, 2013, 01:06 PM   #11
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Doesnt spikes tactical make a good bcg? Can be found for about $140.
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Old August 9, 2013, 08:20 PM   #12
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Spikes make a good BCG.

The really crappy ones to look out for are the BOLTS made from 8610. PSA sells bolts made from C-158 and cheaper ones made from 8610 so pay attention when ordering. )
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Old August 9, 2013, 10:41 PM   #13
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Noted. Is there a big difference between the chrome BCG's and the nickel boron BCG's? A lot of the places that sell the NiB ones say that it is stronger, but is it actually noticeable in the performance/longevity of the weapon?
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Old August 9, 2013, 11:15 PM   #14
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M4Carbine.net isn't so bad...you just have to understand that you're not talking to Bubba Plinker when you're there....answers can be blunt, but are generally correct...even if they are not what you want to hear.
The general consensus on the NiB coatings that I see is that they make cleaning the BCG a little easier, but don't add anything when it comes to strength of longevity.
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Old August 11, 2013, 09:31 AM   #15
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Plouffedaddy's everything you ever wanted to know about AR BCGs video link

I get this question a lot so I made the video below explaining all the specs, what they mean, why they're important, ect....
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Old August 11, 2013, 02:45 PM   #16
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Thanks for that video, it was very informative.
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Old August 12, 2013, 08:40 AM   #17
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Thanks for that video, it was very informative.
Thanks. I shot a barrel and buffer video too I just haven't uploaded them yet. They're just such common questions out there---someone needed to answer them... Good luck with the rifle!
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Old August 12, 2013, 09:44 PM   #18
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I'll definitely keep my eye out for those videos as well. I'll post on here again when I complete my build. Right now I'm just waiting the 10 days CA makes me wait to receive my lower receiver.
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Old August 22, 2013, 12:36 AM   #19
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Finally got all the parts together and finished my build. All the internals are Spike's parts coated in Nickel Boron. The handguard is from Fortis and the covers are from Noveske. Most of the rest of it is from a local brand called JD arms. I spent about 900 bucks on it.

It was fun, I learned a lot, and I can't wait to start my next build.
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Old August 23, 2013, 08:25 AM   #20
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Personally, I would pay the extra money for chrome lined, or at least NiB. Sure it's overkill, but its good overkill. If it's good enough for full-auto, it's even better for semi-auto.
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Old August 25, 2013, 09:51 AM   #21
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Some good and some bad info will always come up when talking about BCGs. Even plouffedaddy's video, which most are pretty good, has several errors in it.

For instance, shot peening does NOT increase the pressure rating. The actual reason for shot peening is to eliminate smooth and deform small surfaces cracks and crevices created in the machining process. The cheaper (not mil-spec) bolts don't benefit much from it, but the harder the steel, the more the benefits. The shot peening does result in a slightly harder surface, but the smoothing of the cracks reduces the propensity for corrosion cracking and mitigates the stress concentrations.

Also, a FA carrier does not reduce the stresses at all, in fact it may increases them. For high performance, lightened BCGS are preferred. Youngs and JPs are the only two with an enlarged rail surface which effectively reduces the wear of the receiver. Spike's tried one, but left the rails the same size, total fail. The mating of the gas system to the weight of the buffer and BCG are important, and something most people don't consider, much less understand.

The four major failure areas for BCGs are extractors, bolt lugs, leaking gas keys and gas rings.

Lets look at extractors first. The spring, insert and doughnut are what results in extractor failures. The gold Colt spring with the black insert is the best bet, and any bolt can be upgraded to this for about $8. If $8 is too much for you, then run an o-ring. The o-rings really just increase the lifetime of the spring a bit, but the springs, inserts and o-rings, whatever you use, are maintenance items that should be changed out, maybe 10K or so.

When it comes to bolt lugs, they primarily shear due to a dull cutting tool and either no, or not enough shot peening. The enhanced bolts that use better steel do last longer and if properly shot peened, is a good $40 or so for a bolt that will last longer than most people will ever need.

A properly torqued and sealed gas key is better than a staked gas key. I have seen plenty of gas keys leak and when we tear them down, the bolt has been sheared, but the staking has held the bolt head in place. Some people get too aggressive and put a side load on the threaded connector (which it was not designed for) and after several thousand rounds, the bending load on the thread root results in cracking. The two indentations on either side of the bolt head must be symmetrical, perpendicular to the carrier axis and not have material flow up and over the top of the key. Most of the "see, this is properly staked" photos I see really are not done right. Don't get too carried away chasing "mil-spec". I have never seen a properly sealed and torqued gas key fail but I have seen plenty of "mil-spec" staked keys fail.

Gas rings are a wear item. The dynamics of your gas system (length, gas port size of barrel, buffer weight and carrier weight) all affect wear. The more gas you put in and the heavier the reciprocating weight, the faster the gas rings will wear.
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Old August 25, 2013, 09:49 PM   #22
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MarkCO is clearly speaking from experience. Everything he says makes sense to me.

Speaking from as a fatigue engineer with 25 years experience working with military platforms... hear are my thoughts on the BCG...

Some of the testing aspects of the Mil-Spec are simply to save the military the need to test fire the component. For example, a high pressure test of the BCG followed by MPI is designed to reveal manufacturing flaws that would lead to a short fatigue life. However, you or I could achieve the same thing by running 500 rounds or so through the rifle. Any initial flaw would reveal itself by then. Once a bolt has 500+ rounds on it, it is no different than an MPI'd bolt. Both have been tested to reveal inherent flaws in the steel. For the military, it makes sense to inspect the bolt with an MPI technique. They don't have the luxury of firing hundreds of rounds at the range.

Shot peening the BCG is an odd practice in my opinion. Rarely is shot peening called out as part of an original design. It is usually a Band-Aid to address a fatigue problem that comes up in the field, but in my experience, the military never allows shot peening to be the only solution to a problem. They will demand a substantive fix, and the engineer will usually have to "show it good" without the shot peen. So I would bet money that at some point there was a fatigue issue with the bolt, and a design change was made... could have been a change in material or heat treat, could have been a change in geometry... and the engineer (probably at Colt) had to prove the design change fixed the problem... Oh and by the way, they threw on some shot peen for good measure. It's like belt AND suspenders.

The shot peen needs to be done before the proof testing. It is a fatigue no-no to shot peen a surface that has been exposed to proof loads. If a crack was started by the proof loading, the peening will not make it go away, but it will make it impossible to find. The crack will likely still propagate as a sub-surface crack. So peening needs to be done before any loading.

Carpenter 158 is an interesting choice for a steel bolt. It is a plastic molding steel... It is used to make die cast or injection molds for plastic parts. Evidently it works pretty good for rifle bolts as well. But don't ascribe any magical properties to it. It is just a good quality tool steel. Nothing more nothing less.

The oddest (and most bizarre) feature of the mil spec BCG is staking of the gas key fasteners. "staking" is a low tech and unsophisticated way of providing anti-rotation of fasteners. There are a lot better ways to do it. You won't find very many staked fasteners in a modern automobile engine, and you will never ever find them in aviation. A high temp thread adhesive would work much better in my opinion. But the one thing that staking has going for it, it allows a government inspector to know at a glance that those two fasteners have been staked. And I strongly suspect that this is exactly why those two bolts are staked... it makes the anti-rotation feature inspect-able.

If you have an AR with a non-mil-spec BCG, my advice is to put 500 rounds through it, remove the BCG and clean it up and put it in a Ziploc bag. Then put in a new BCG and put 500 rounds through it.... Now you have a known reliable BCG in the gun, plus a spare that you know to be reliable. The BCG represents half or two-thirds of the likely field problems an AR might have (other than magazine issues), and BCGs are super easy to replace. You are good to go.
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Old August 27, 2013, 10:20 AM   #23
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That is great information. I like learning about little details like that. It helps me, and anyone who reads this thread, make a better purchasing decision. What you both said about the staked gas key makes a lot of sense. My only qquestion about that is where would I find a BCG that lives up to those standards? It seems as though nearly every major "reputable" company stakes their gas keys. Also, if you could expand upon the importance of the mating weight of the buffer and bcg to the gas system, that may help me greatly on my next build.
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Old August 27, 2013, 12:05 PM   #24
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Young's properly seals their BCGs and does not stake them. You can also buy a stripped carrier and properly torque and seal one yourself.

Your second question is more complex.

For a given load in a given barrel, there will be gas volume and pressure curves. The shorter the gas system, the higher the pressure and temperature of the gas that enters the gas port of the barrel. The length of the barrel beyond the gas port has to do with dwell, or really area under the curve. Fundamentally, there are thermodynamic equations that are well beyond the capability of most people to understand, so I try to keep it in the lay verbiage.

Think of the barrel gas port as a valve that opens when the bullet passes it and the bullet as a piston in an expanding cylinder. The gas pressure is eroding, but the gas volume is increasing. When the bullet passes the crown, the pressure drops and you have no more volume going into the gas tube. At that same time, the case springs back and can be easily extracted. The mass of the carrier and buffer do somewhat act against the rearward movement while the bolt is still locked into the barrel extension. The gas pressure has to fall for the bolt to unlock. Try to extract, or unlock, too early or too late and you will have problems. Most heavy gas ring wear, failure to extract and eventually failure to eject or even chamber is due to improperly timed gas systems. Many times the extractor groove and firing pin channel fill with material due to the improper timing and resulting in other failure modes. Sure, there is a big window, but get outside of that window and you will have problems, some only manifest by breakage down the road.

Realize that pressures drop about 7K to 8K for each subsequently longer gas system, but if we don't have enough dwell, we will be undergassed volumetrically.

Also, realize that the "original" system was designed over 50 years ago. Our primers, and powders are now more efficient, and some are hotter and you can start to see the recipe for increased wear, and increased failures if we do not alter masses and or gas port sizes.

The vast majority of ARs are overgassed well beyond the necessary window. That window has to account for chamber fouling, temperatures, different case material, chamber dimensions, etc. for reliability. The proliferation of adjustable gas systems, for both suppressed and un-suppressed variants, is due to these issues, but it also introduces a new failure mode. Reduction of the gas port size is the best first step, but most companies do not realize this, or they know the mil-spec fanboys will stop buying their products if they reduce the gasport sizes. Mid-length gas systems on the 14.5 and 16" barrels also help with this issue. If you reload, you can also address the issue.

Heavier carriers and buffers can be used to mitigate early unlocking, but with a corresponding increase in the 3rd and 4th pulse of the recoil cycle. You increase the dwell time of the expanding gasses in the carrier and therefore you also increase wear on the gas rings. The 3rd and 4th pulses of the recoil cycle are the two that really affect the muzzle flatness. With longer gas systems, we can get away with reduced mass which helps flatten out the gun.

I hope that helped some.
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Old August 27, 2013, 12:38 PM   #25
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I hope that helped some.
Very well written: clear and concise.

I stick with the original rifle layout and buy parts when they go on sale rather than trying anything unusual or expensive, but these explanations were still interesting and informative.

EDIT - Ditto, btmj.

Last edited by zukiphile; August 27, 2013 at 12:45 PM.
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