Help me understand bolt carriers AR-15


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ohbythebay
May 4, 2014, 09:39 PM
I am looking at Bolt carriers for my S&W M&P 15

I ran across this
Centurion Arms C4 Bolt Carrier Group was created with MPI tested bolts and marked to give you the ultimate confidence in your weapon system. This full auto carrier is heat treated and precision machined. The gas key is staked on both sides of both bolts

When they say full auto do they mean spec'ed for it or do they really mean full auto ?

Also, as I understand it - some are M16 spec'ed to be a bit more robust, yes ?

Thanks in advance...

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taliv
May 4, 2014, 09:43 PM
Do a google image search and you will find comparisons that will be easy to understand.

The auto carrier has a circular back end that is designed to trip the auto sear in a full auto Lower.

The semi auto is more like a half circle and won't trip the auto seat in a lower.

Most people prefer auto seat in semi auto because the extra weight in theory slows the cyclic rate a tiny bit by making the chamber unlock a tiny bit later and the extra weight you forward makes it a tiny bit more reliable overcoming a tiny bit more gunk induced friction.

In theory

taliv
May 4, 2014, 09:44 PM
Stupid iPhone autocorrect

s/seat/sear

jerkface11
May 4, 2014, 09:45 PM
And if your gun breaks and the hammer follows the bolt home a full auto carrier won't let it hit the firing pin.

ohbythebay
May 4, 2014, 09:47 PM
So besides the shape, it will work and not convert my rifle to auto (which I didn't really think) but wanted to be sure. I don't really NEED a new BCG...at the most maybe keep a spare bolt..but might be a fun upgrade...but other than durability, I am not really going to notice a difference am I ? :rolleyes:

rcmodel
May 4, 2014, 09:56 PM
No, you will not notice a difference, if your old one works as designed.

carrier is heat treated and precision machined. The gas key is staked on both sides of both boltsThats just windy add copy.
All bolt carriers are heat treated and precision machined.
And all gas key bolts should come staked in place.

In other words?
If your bolt carrier ain't broke, don't fix it!

rc

ohbythebay
May 4, 2014, 10:24 PM
Sort of like "soft Corinthian leather..."...lol

I do like this one though...
https://www.rainierarms.com/?page=shop/detail&product_id=3653

And it aint broke and just in mind if I do fix it..LOl

Robert
May 5, 2014, 12:33 AM
All bolt carriers are heat treated and precision machined.
And all gas key bolts should come staked in place.
Eh, not so much. There are lots of crap parts out there.

But your S&W is not among them.

Onmilo
May 5, 2014, 12:35 AM
http://www.fototime.com/C466D2CD5713778/standard.jpg
This image might explain it best
Top carrier is a semi auto type lightened carrier.
The rear lug of the carrier is cut back and the firing pin port shroud is cut away.
This prevents use in a full auto converted weapon as the cut away lug makes it impossible to add an auto sear so just modifying the disconnector will cause the carrier to trap the hammer in the clearanced firing pin port during full auto fire and effectively jam the weapon.
At most, the weapon will only get off two or three shots before jamming in epic fashion.

Middle carrier is a full auto type, notice the full length rear lug and shrouded firing pin port.

Bottom is a standard Match type semi auto carrier.
It features a shrouded firing pin port that allows the use of Match type two stage narrow hammers as it will not trap the hammer in the clearanced firing pin port and the rear heavy lug has been cut back to prevent the carrier from contacting an auto sear and allowing full auto firing.

None of these carriers are considered machinegun parts by and of themselves and as stand alone parts may be added to any semi auto weapon without repercussion HTH

ohbythebay
May 5, 2014, 01:08 AM
And also...the picture was worth 1000 words...makes sense...

I like that middle carrier btw..who makes it ?

dvdcrr
May 5, 2014, 01:11 AM
Stupid iPhone autocorrect
You can turn that off.

pyrex
May 5, 2014, 01:26 AM
all correct, however the top and bottom carriers would never even fire 2-3 rounds in auto. The hammer would drop, fire the first round, the hammer would catch on the auto sear and then nothing would trip the sear so the gun would effectively stop functioning. You'd then drop it to semi auto and it would release the hammer off the auto sear and onto the disconnector, allowing it to function again. Nothing fantastic or spectacular.

Arizona_Mike
May 5, 2014, 03:27 PM
Just about anything form Colt in recent years is M16. I'm partial to the BCM BCG (https://www.rainierarms.com/?page=shop/detail&product_id=3653).

Mike

JoePfeiffer
May 5, 2014, 03:59 PM
Of one were building an AR for plinking/coyotes out to about 300 yards, which type would be preferred?

ohbythebay
May 5, 2014, 04:23 PM
I like the BCM also...

https://www.rainierarms.com/?page=shop/detail&product_id=3653

Sort of came to the same conclusion. The one on my S&W is fine...but I might (haven't decided) get the BCM, use that and keep the original factory as a spare. I think the factory carrier is great..not sure I am as impressed with the bolt itself. We'll see ! Lots of other toys I need to buy first. I keep going back and forth/scope/no scope...

taliv
May 5, 2014, 04:44 PM
Of one were building an AR for plinking/coyotes out to about 300 yards, which type would be preferred?

coyotes won't notice a difference.

it's part of a system. the barrel length, gas port location, gas port size, buffer weight, action spring, etc all work together to determine how the action cycles.

generally speaking, if you don't know what you're doing and have a really good reason for doing it, you shouldn't mess with it. that said, there's about a 99.9% chance you won't be able to notice a difference when swapping full auto and semi-auto carriers back and forth.

Arizona_Mike
May 5, 2014, 07:59 PM
I like the BCM also...

https://www.rainierarms.com/?page=shop/detail&product_id=3653

Sort of came to the same conclusion. The one on my S&W is fine...but I might (haven't decided) get the BCM, use that and keep the original factory as a spare. I think the factory carrier is great..not sure I am as impressed with the bolt itself. We'll see ! Lots of other toys I need to buy first. I keep going back and forth/scope/no scope...
Thanks for the Rainier link. I'd rather order from them as they are such a great company. I love their barrels and their customer service is good. when they screw something up they make it right.

Mike

Arizona_Mike
May 5, 2014, 08:00 PM
Of one were building an AR for plinking/coyotes out to about 300 yards, which type would be preferred?
Coyotes don't care but for recoil and reliability the heavier M16 carrier is always the better choice.

Mike

C-grunt
May 5, 2014, 08:42 PM
I took an AR15 armorers course last weekend. In that class over half of the class had improperly staked bolt carriers. You could remove the carrier key screws with a hex head driver with not much effort. Many showed signs of gas leakage under the key.

OP... look up what a proper staking looks like and make sure yours is. Being a S&W it probably is.

back40
May 5, 2014, 11:06 PM
The one on my S&W is fine.

if you have a reliably running ar with a properly staked key, why would you consider swapping carriers? what do you hope to gain over the reliability you've already experienced?

Onmilo
May 6, 2014, 12:25 AM
At the minimum, staking should look like this,
http://www.fototime.com/C9F3E39F401E57E/standard.jpg
Enough material displaced against the sides of the cap screws to grip and the screws should be torqued to at least 45 INCH/Pounds which is just shy of three FOOT/Pounds.
There is no need to hammer peen the screws in place nor is there any need for any kind of glue, i.e. LocTite, if this is done correctly.

justice06rr
May 6, 2014, 06:14 AM
Buy that BCM BCG if you have the extra money. BCM is a very good company and highly respected in the AR world.

Nothing is wrong with your current S&W BCG, but it never hurts to have a spare esp if its a BCM...

AlexanderA
May 6, 2014, 01:00 PM
A bit of historical background -- the first semi Colt AR-15's (that came out circa 1964) had bolt carriers like the bottom one in Onmilo's picture, above. By the late 1960's the idea sprung up that you could convert these to full automatic simply by removing the disconnector (in reality, you'd just have "hammer follow-down" and have uncontrolled doubling with soft primers). Anyway, ATF put pressure on Colt to stop selling these, and, indeed, around 1970 Colt pulled the AR-15's off the market for a time, pending a redesign of this feature. The solution Colt came up with was to "unshroud" (bevel) the bolt carrier (top one in Onmilo's picture) and use a "notched" hammer. Therefore, if the disconnector was removed, the notch in the hammer would catch the flange of the firing pin and the gun would hopelessly jam. (And BTW, an "unshrouded" bolt carrier has to be used with a small-diameter firing pin or the gun will jam regardless.)

Later, this was found in practice to not be much of an issue and Colt returned to "shrouded" bolt carriers. Later still, Colt went even further and got ATF approval to use full-auto type bolt carriers (middle one in Onmilo's picture) in semi rifles, and that's the situation today.

(As an aside, during the period in the early 1970's when the Colt was off the market, rival Armalite introduced the AR-180, which found its niche because it was the only "black rifle" available to civilian shooters.)

goon
May 6, 2014, 06:33 PM
If your S&W runs, I'd leave it alone. In theory, an FA carrier is better for the reasons discussed above. In practice, if the rifle is reliable, don't look for a reason to spend more money on something you don't need.

RussellC
May 6, 2014, 11:40 PM
Check this out to find most all you questions answers:

http://forums.officer.com/t81462/

Best explainations an all things AR I've seen. You will have to scroll down a while to get to the bolt carrier info...

Russellc

Tirod
May 8, 2014, 01:10 PM
One difficult area of interpretation is using the M4 TDP as a guideline to buying parts for a carbine that we are building. And we aren't going to war and will not be allowed to bring our own guns.

Where the milspec part may be adequate in protecting the taxpayer's money being spent, it was necessary because the guns are built by the lowest bidder. Features on a military weapon that the gov't insist be available aren't necessarily your optimum choice for a hunting, sport, or competition rifle that may actually see a lot more use, or require a lot more precision.

Take it all with a grain of salt, read up on various AR forums, and decide for yourself.

C-grunt
May 9, 2014, 12:05 AM
The screws need to be staked. They are only in with 3 ft lbs of torque. If they back out your rifle turns into a single shot. I've also seen where a loose screw broke leaving half of it in the bolt carrier. I bet that was fun getting out.

Staking a carrier key is easy. If a company doesn't do it they are just plain cutting corners.

Frosty Dave
May 11, 2014, 06:43 AM
Some of the posts in this thread point out another largely unanswerable question - how do you know the manufacturer you're buying really follows "mil spec" and how do you know the advice you're getting on the internet is any good? The specification for gas key screws is actually 50 - 58 inch pounds, or about 4.5 foot pounds.

Bexar
May 11, 2014, 08:30 AM
Can't vouch for this guy but he does cover some of the questions. I think he has a show on one of the cable channels.

https://www.youtube.com/watch?v=4X_D6UH3dLM

taliv
May 11, 2014, 06:13 PM
Frosty that is good point. I believe very little of what is posted unless it is by someone I know. Then I am able to evaluate their statements in the context of their background and previous statements.

We don't know if companies follow any spec. Even if they say they do. That is why auditors do billions in business annually. But there are people out there who watch millions of rounds per year go downrange or who are involved in testing etc who collect enough data to have a meaningful opinion.

Warp
May 11, 2014, 11:13 PM
One difficult area of interpretation is using the M4 TDP as a guideline to buying parts for a carbine that we are building. And we aren't going to war and will not be allowed to bring our own guns.

Where the milspec part may be adequate in protecting the taxpayer's money being spent, it was necessary because the guns are built by the lowest bidder. Features on a military weapon that the gov't insist be available aren't necessarily your optimum choice for a hunting, sport, or competition rifle that may actually see a lot more use, or require a lot more precision.

Take it all with a grain of salt, read up on various AR forums, and decide for yourself.

Incorrect.

You need to complete that sentence for it to be correct. ;)

Onmilo
May 12, 2014, 10:08 AM
Frosty Dave
If your accusation is directed to me, read the post again.
"At LEAST 45 INCH Pounds."
I did not specify a maximum.

Most guys will overtorque and overpeen and add a big dollop of Loc-Tite to the mix thinking more is better.
It isn't.

By the way.
At 60 inch pounds a hex bolt can be easily removed with a good quality Allen wrench.
If the staking is done correctly you need a cheater pipe added to the wrench to break the bolts free.
It's done this way for this reason.
So you CAN remove the gas tube key if you should ever need to, i.e., the gas tube port splits or the gas tube batters the port because it is misaligned, etc.

If you can't remove the bolts with a good Allen wrench and a cheater, you way overdid the screw down job.

That "gas leakage under the gas key" is a bunch of hype.
EVERY gas key I ever pulled off a bolt carrier while in the Army had a bit of carbon fouling, "Gas Leakage" under the key.
Oh, every one of those rifles are built to "Mil-Spec" too.
Nature of the beast.

Frosty Dave
May 12, 2014, 11:10 AM
Onmilo, that was not directed at you but I see how it seemed that way and I apologize for not wording it differently.

It was indeed your post that brought the topic to mind. Had I been a little more verbose and attentive I might have better made the point that gas key/carrier assembly is one of several points of AR technical lore where the advice seeker can get many facts and opinions, some of which are sound but may range all the way to voodoo.

Tirod
May 12, 2014, 12:42 PM
Specs and inspection standards are always an interesting issue. I have worked for a government supplier, who was required to document each step of construction, in which a worker initialed off the work was done to standard. And a government inspector was on site checking, and they would fail a lot on occasion. For a hundred piece lot, there could be anywhere from 15 to 50 pages of documentation.

Said documentation accompanies the lot to it's first distribution point. In the case of the M16 and M4, that is handled by the receiving location under Tank and Armament Command, who inspects the weapons and paperwork, then signs off as being compliant to standard.

Technically, until that is done, it's not milspec, and only US Govern't guns accepted for service are. All others are not, even if made with the identical parts. No paperwork, it didn't happen, and it's not official unless accepts. Military spec requires government acceptance as the result.

On the open market, we buy the seller, and do that by judging from their text in ads, the maturity of their marketing image, and their ability to sell firearms that are accepted under government standards.

So, Colt, FN, Remington, Bushmaster, LWRC, and a few others are "milspec" providers. Nonetheless, someone could assemble or purchase a Noveske, BCM, Armalite, or half a dozen others and have a better gun, more reliable, tougher, more accurate, and more ergonomic with the proper accessories. The Marines use the Norgon ambi control, the Army, not so much.

In general, the leading "best practices" for making a firearm are often more expensive, and are not used unless required by the TDP, milspec, or are just cheaper, which isn't often. Therefore, what gets bid for contract is often the lowest possible cost expense for the manufacturer. They don't gild the lily with CNC billet lowers or Noveske keymod rails. Even Colt doesn't own a forge, they buy the platters from one and machine the lowers and upper from them. I seriously speculate that a lot of smaller parts aren't made in house either - springs, screws, etc. are expensive to make on a contract basis in house, and the existence of a lot of the manufacturing base outside the company walls of a lot of makers is evidence. In fact, there are very few who advertise that they make the entire rifle right down to the last screw. What you do hear is that they have the highest quality, tho, and it's always in serious prose with no fooling around. If they refer to the user base and it's professionalism, they don't indulge in popular catch words and keep it oblique.

I haven't seen an HK ad with their gun and Osama's last photo op proclaiming they are the Gun That Rules Them All. On the other hand, there was a lot of personal bragging from a highly placed government official taking the credit for the kill. . . .

So, the rule is the trashier and more jingoistic the ad, the less likely that maker actually puts out a product that is gov't spec. They are selling a lot of sizzle to hide the fact their steak isn't all that. It's marketing 101, if you aren't all that, dazzle them and baffle them.

It really is a matter of interpreting nuance and reading between the lines. It takes some skills on the internet in reading comprehension and understanding the source that information is coming from.

I will say it, tho, that even a qualified and experienced user will get it wrong. They are not a firearms designer - User 1 didn't make the machine, and no amount of bluster on a forum guarantees they have a complete understanding of it's dynamic cycling. They didn't do the gas pressure analysis, don't work in a plant making them, and didn't come up with a lot of the modifications. In most cases, they weren't even born when it was invented. Be very careful taking away any more from it than what their subject matter expertise has taught them - like a MD who spends half his week on construction projects, having an opinion and forcefully expressing it doesn't make them an engineer.

Therefore, listen carefully and double check facts before accepting them. If everything that was said on the internet was true, then it would be an interesting world indeed.

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