When were 9310 bolts first introduced?
Early on there were some issues with properly heat treating that led to breakage. Has that since been corrected? If so when?
Some say a properly heat treated 9310 is better than mil-spec C-158?
C-158 bolts are available in both phosphate and nitride, it appears all 9310 bolts are nitride only?
Has Crane tested 9310 bolts with the latest heat treatment processes?
Are there any drawbacks to a properly heat treated 9310 bolt?
Are 9310 bolts still considered unproven?
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The first bolt made from 9310 I know of was made sometime around 1964.
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There were? I never heard of wide spread problems.
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And, some say the opposite. Everybody has an opinion.
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This is more of a statement than a question.
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Why should NSWC Crane want to test bolts?
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They aren’t “MIL-SPEC” for the M16/M4.
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Bolts made from 9310 have been used on M60s and M249 for many decades without any issue. According to a 1962 report by Watertown Arsenal, 9310 steel was not only more tolerant of heat treating condition variations than the then ‘standard’ 8620 steel, it was superior in strength. For these reasons the 9300 series steels have been very popular in weapons breech bolts.
Hodge Defense uses a 9310 bolt in their ARs so the concept is proven. Just like any other gun product, if they’re made right they’re good. If they’re not, the forums have more to talk about.
- What I have read led me to believe 9310 was a newer metal.
2.The only issues I have read about concerning 9310 bolts were premature breakage in the AR/M4
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What is your opinion?
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Why is Mil-Spec phosphate instead nitride?
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I am aware NSWC Crane tested the mid-length gas system. Just curious if that type of testing is common for vetting newer potential improvements?
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Why not?
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I did not this. Why was C-158 chosen for the M16/M4?
Carpenter 158 was originally developed as a dies steel and when carburized is best suited for application requiring high core strength and subjected to heavy shock and wear.
Carburized 9310 is a very good steel when high core strength, toughness, fatigue, and wear properties are required. It’s biggest use is in heavy duty gears and crankshafts.
In 1962 Watertown Arsenal published a report outlining the superiority of AISI 9310 over the “go-to” gun steel: AISI 8620. Shortly thereafter Springfield revised the drawing for the M60 GPMG bolt listing AISI 9310 as the material. When the FN Minimi was designed in Belgium they chose something that was the equivalent of AISI 9310 for the bolt in that as well.
Differences between the two alloys are rather small, the only real difference is the addition of 0.10% of molybdenum in 9310, the phosphorous and sulfur are basically impurities left over from smelting, and Carp 158 would have some of these impurities, even though their alloy listing doesn’t mention them. Both alloys can attain core strength in the 160 KSI range and have similar shock, toughness, and fatigue properties
Carpenter 158 Composition (%):
Carbon - 0.10
Chromium - 1.50
Manganese - 0.50
Nickel - 3.50
Silicon - 0.30
Iron - balance
AISI 9310 Composition (%)
Carbon - 0.08 to 0.13
Chromium - 1.00 to 1.40
Manganese - 0.45 to 0.65
Molybdenum - 0.08 to 0.15
Nickel - 3.00 to 3.50
Phosphorous - 0.025 max
Silicon - 0.15 to 0.30
Sulfur - 0.025 max
Iron - balance
So, if all things are equal for the most part between Carpenter 158 and AISI 9310, why only Carpenter 158 for military spec parts? Mainly, the military (specifically the Army) does not now, nor ever did own the design for the AR-15 bolt, and because of this, Colt’s Manufacturing controls the material specifications. If the Army felt there was a problem in getting Carpenter 158, or felt there was something significantly better, they could request Colt change the specifications, but that would cost money to: a) research and prove out an alternate, and b) pay Colt’s to change their drawings. It may very well be that 9310 is a superior steel for M4 bolts, but the cost to do (a) and (b) are higher that the savings you might recoup from fewer bolts purchased, i.e., no return on your investment. Similarly, Colt’s has no incentive to change, carpenter 158 has worked adequately for the last 60 years, so why spend money to fix something not broken.
So why are other people making 9310 bolts?
Carpenter 158 is a proprietary alloy, and only available from Carpenter Technology. If you want some, you probably have to get a few tons of it. AISI 9310 is non-proprietary and available from many different foundries, in smaller quantities, and probably a lot cheaper. Many AR bolt manufacturers make bolts from 9310, and they work just fine and last just as long, but they will never be “proven” until somebody with really deep pockets reaches into those pockets and funds a serious test of the two bolts. And, the only folks with pockets that deep have no incentive to conduct such a test, because they have something that works, either 158 or 9310.
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9310 steel has been around since the late 1950s.
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I have not heard of 9310 bolts breaking in ARs with any greater frequency than bolts made from Carpenter 158.
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My opinion is in post #5.
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Because the drawing originates from 60 years ago when the available methods for nitriding were either expensive, or prone to leaving a thick “white layer” which does not enhance fatigue properties. And as noted in post #5 changing things cost money, so why do it unless you really have to.
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As noted in Post #5, very low potential return on investment.
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See post #5
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For that you would have to ask the materials engineer at Armalite that recommended Carpenter 158® as a bolt material, maybe he/she didn’t know about Flexor®, or CX®, and didn’t think S-7 would be a good choice.
TL;DR:
- Milspec isn’t the end-all, be-all…it’s a benchmark or a minimum standard…NOT the peak of performance
- Bolt life performance depends on how well the rest of the gun is built; a mediocre bolt will last in a well-built gun longer than a better-than-milspec bolt in a poorly built/gassed/timed gun
- Apples-to-apples comparisons are rare but what is probably true is that well-built and properly tested 9310 bolts are just fine.
- Superior alloys materials exist well beyond “milspec”. Remember the crash test/recall scene from “Fight Club”…or consider the LMT or KAC “enhanced” bolts. Would you rather a “good” bolt that’s gonna cost you $65-75 per bolt or a “great” bolt for $180-250 per bolt?
lysander,
Thank you for the 9310 summary. Quite educational and I appreciate it very much!
Paul
Thank you lysander for continuing to share your knowledge with us on this forum. I know some SMEs have left over the years, and I truly appreciate the folks who’ve stayed. I hope to keep learning more and more as long as I can.
Sent from my Pixel 3a XL using Tapatalk
As to the question of why Carpenter 158 was chosen as the material for the bolt, rather that an AISI standard steel alloy (like 9310), here is something I was once told by an engineer at one of the big aerospace component companies. Manufacturers, especially those in the aviation field prefer to use proprietary standards, rather than institutional or Federal standards if they can. Proprietary standards, for something like an alloy composition, are owned by the company and usually trademarked, thus changing the composition is highly unlikely. Further, with a single source, they have some leverage to keep the composition from changing - “I’ll only be buying from you as long as you don’t change the product.”
Institutes, associations and other such organizations, like the American Iron and Steel Institute, do not manufacture anything and, theoretically, can change their specifications on a whim if they see fit. This could mean that the 9310 bought today is not the same as the 9310 bought next year.
Manufacturers fear not have tight control of their material supply. While this is a logical concern, especially in the aerospace industry, it is really not something I have ever heard of; a specification changing so drastically that it is unfit for applications that previously used it, but that seems to be the way some companies think.
You’re right, but MILSPEC is a specification and even at the minimum, it still meets the standard. The other wonder steels used and special bolts are merely marketing without the thorough and expensive testing mentioned above. MILSPEC bolts have a 60 year track record, 9310 and the other enhanced bolts do not.
In engineering, you have what is known as “suitability by similarity”. If two objects, or materials, are similar enough, you can assess the suitability by examining the similarities and differences (if any). Carpenter 158 and 9310 are very similar steels and in other applications show similar behavior. Further, 9310 does have a long and successful history as a bolt material. I would have to say that 9310 is a perfectly adequate substitute material.
Isn’t the expensive price of the LMT Enhanced bolt partly because of the research and development costs?
Agreed.
I don’t disagree. The reality here is that in the AR platform 9310 has near zero track record compared to carpenter 158. The minute differences between the two isn’t enough to make the change to 9310 any significant advantage if any at all. The hype around 9310 is just that.
double tap.
The hype stems from the amount of circulated misinformation. Even from industry insiders and so called industry “professionals”.
The biggest issues I’ve seen with 9310 is the result of improper heat treat, and 9310 is much less forgiving with the treatment temperature range. 9310 is harder than C158 but it’s also more brittle, it’s fatigue life is also notably shorter than C158.
SOLGW is also not fond of 9310. https://www.arbuildjunkie.com/ar-bolt-carrier-group-basics-mike-mihalski/
Here’s what Bill Alexander stated about 9310
Absolutely this. I work for a AS9100D manufacturer, and project managers and engineers don’t care if there’s better substitutes, they care about what material is certified.
The heat treat issues sited are when 9310 bolts in the AR were initially introduced and they continue to brought up again and again.
Is there any recent data that 9310 bolts are still failing more than C158?