The Geissele Stressproof Bolt is made from Carpenter 158, however this is not your typical material spec’d by the US Government. Since Geissele wanted to take it to the next level, their engineers worked directly with the Carpenter Steel metallurgists in Reading PA, to produce a special heat of material known as Carpenter 158+. This material is cleaner with less impurities, ultimately making it stronger and more consistent. They did not stop there, they decided to forge the bolt. Using the same process used to produce upper and lower receivers, a forged bolt manipulates the grain structure of the metal and yields a bolt capable of 5 times the life of a mil-spec bolt. Each bolt is then rigorously inspected, high pressure tested, mag particle inspected and coated with Nanoweapon for maximum corrosion and wear resistance.
Nanoweapon is a family of coatings similar to DSL (Durable Solid Lubricant) that was developed by Picatinny Arsenal (US ARMY ARDEC), and is only available exclusively from Geissele Automatics. Their engineers worked for over 3 years with the research and development company that worked with Picatinny to develop the coating in order to fine tune it into what it is today, making it the pinnacle of coatings for firearm components. No other coating on the market can provide the same level of corrosion, wear, and abrasion resistance as Nanoweapon. The coating is applied at low temperatures so it does not affect the metallurgy of the part, and has a surface hardness equivalent of 82HRC. At that hardness, it is harder than sand, easily rejecting carbon and making cleaning a breeze, on top of allowing the firearm to operate normally with less lubrication.
I have a number of reports that stated that 9310 is actually a very forgiving steel to heat treat compared to say 8620 which no one has every accused of being a difficult steel to treat. If fact, 9310 response to thin section quenching is better that Carpenter 158.
And, taking the leap between this statement:
“. . . by comparison to Carpenter 158, AISI 9310 has several elements present in its composition that are detrimental to fatigue while not being evident in the physical properties . . .”
and this statement:
". . . 9310 is harder than C158 but it’s also more brittle, it’s fatigue life is also notably shorter than C158 . . . "
Is an unsupportable leap.
First, I am going to take issue with “several elements present in its composition that are detrimental to fatigue”, the only element in 9310 that is not in 158 is molybdenum which is not “detrimental to fatigue”, the other two elements , as stated earlier are impurities that will be present to some degree in all alloys.
Second, 9310 is not “harder”. It is hardened to achieve the proper strength, and that happens to be the same hardness as Carpenter 158, around 36-40 HRc.
And last, I have not seen any study of 9310 that states it has a short fatigue life. In fact one of the reasons it is popular as a gear material is because it can withstand high cyclic loading over a long period of time.
So, stop requoting the myth that 9310 is “harder to heat treat”, “is harder and more brittle”, and unless you can cite a study that shows 9310 has a poor fatigue behavior, stop that too.
“Low Cycle Bending Fatigue of AISI 9310 Steel Spur Gears”
In a fatigue test of AISI 9310 steel gears the information shown below on cycles to crack initiation was reported. The steel in question was carburized at 1650° F for 8 hours, austentized at 1550° F for 2.5 hours, quenched in oil, frozen at -120° F for 3.5 hours and tempered at 350° F. Since 9310 AR bolts are proprietary, I can’t say what process they used to heat treat their bolts, but the above is almost exactly the heat treatment used for the M60 bolts.
In “Failure Analysis of an M16 Rifle Bolt”, the calculated maximum stress in the most heavily loaded locking lugs is shown below:
The red area in the model is 1070 MPa, or around 156,000 psi.
Between these two bits of information, I would say that expecting a 9310 bolt to last at least 5,000 cycles (which is about the earliest point where Carpenter 158 bolts first start to show cracks), is not unbelievable.
I would add the government doesn’t like adding cost to a per-unit item price, whether that is the price of an individual replacement bolt or a complete rifle – unless they are looking for a specific improvement. I have no idea if there is a price delta between Carpenter-158 and 9310 bolts.
Something else (and I don’t know whether or not it matters) to consider – isn’t Crucible the only foundry that batch-makes Carpenter 158?
I would add the government doesn’t like adding cost to a per-unit item price, whether that is for an individual replacement bolt or a complete rifle – unless they are looking for a specific improvement. I have no idea if there is a price delta between Carpenter-158 and 9310 bolts.
The military owns and consumes things – rifle bolts, ammo, people, fuel, etc. If a bolt physically fails at the end of one, two, or three individual barrel service lives (or routine inspection and gaging), chuck it and replace. Done. Machines don’t last forever. It’s why maintenance contact teams inspect, gage, repair, and replace when units rotate back from overseas.
Something else (and I don’t know whether or not it matters) to consider – is Crucible the only forge that produces Carpenter 158?
I remember a conversation with Will Larsen years ago about minimum order size regarding Carpenter 158 steel. It was one of those late night talks as Will was driving across country to teach.
I think you are right, but it was really late at night so I’m not positive. I’ll see what I can find out. I recall him saying availability has sometimes been a challenge, due to minimum order quantities.
I also deleted a couple of dupe posts - sorry for the website lagging at times.
I love this thread. Many thanks to the participants.
M240 bolts were never 9310. The M240 bolt is actually two parts, the breech block itself, and the locking flap. Both are specified in a Belgian steel specification, but these steels are similar to AISI 4130 for the breech block and AISI 8620 for the lock.
It cost about half a million dollars for the Army to change a drawing. The return on investment needs to positive in three or five years, let’s say five as it makes the math easier.
A bolt cost the Govt about $40, and let’s say they buy 100,000 a year, and let’s assume that 10% of that is the raw material. So, in order for this to meet the return on investment requirement, the raw material would have to be 25% cheaper.
Carpenter 158 is not that much more expensive than 9310 . . .
LMT spent a lot of their own money to offer the world a super-duper improved M16/M4 bolt and carrier. Per Karl Lewis, the improvements don’t provide overwhelming superior performance or extended life over the legacy bolt and carrier group – or, rather, enough for the .mil to replace all they have, and to start buying them as replacements vice what’s tested, spec’ed, and in the system and drawings.
So is there actually a way to get the benefits of a hard nitride surface while still retaining the desired core hardness? On a bolt, specifically.
I had the impression that the nitriding is done in a high-temperature (~1000 F) salt bath after the primary heat-treat, and that’s hot enough to temper the core down to something softer than spec. How can this be avoided? Nitride first?
Apparently pre-2005 there were enough M16/M4 C158 bolt failures to conduct these tests.
Failure analysis of the M 16 rifle bolt
V. Yu, J. Kohl, +3 authors M. Veach
Published 2005
Recently, there have been several occurrences of failure in the bolt of the Ml6 rifle at a United States Army installation. Near the failure location, the bolt was subjected to repeated loading as the Ml6 was fired. In order to determine the stress distribution of the bolt due to the firing process, a geometric element analysis was performed using ProMechanica . The fracture surface was examined using both an optical stereomicroscope and a scanning electron microscope in order to determine failure initiation and failure mode. It was discovered that the fracture initiated at a localized corrosion pit and propagated by fatigue. A controlled experiment was conducted where 1800 and 3600 rounds were fired using new bolts. After 1800 rounds, a region of wear was observed near the site where fracture occurred in the failed bolt. After 3600 rounds, a notch was observed in this wear region. This suggests two possibilities: firstly, exposure of the base metal may have facilitated the formation of the observed corrosion pits; and secondly, the presence of a notch may facilitate the fracture of bolts in general. In addition, Vickers microhardness profiles were taken on cross-sectional areas near the fillet region and 10 mm away from the failed locking lug. Disparities between microhardness profiles near the fillet region and 10 mm away from this region revealed that the bolt may not have been uniformly case hardened. Published by Elsevier Ltd
In order to achieve the required strength, you can’t just plunk a piece of steel that has been tempered below the nitriding bath temperature, that goes for a nitrided Carpenter 155 or a nitrided 9310 bolt. As to “how”, you will have to discuss that with a nitriding specialist.
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