9310 vs C158: Is One AR-15 Bolt Really Better? (No)

A lot of "AR-15 experts" who proudly declare their uncontested knowledge of the AR platform are about to get  upset. That's too bad, because those people are, objectively, wrong on this matter. Here's the truth:

When it comes to AR-15 bolts, there is zero meaningful difference between a 9310 bolt and a C158 bolt.

Cue the angry keyboard tapping. 

We're going over what the real differences are between the 9310 and Carpenter 158 steel alloys used to make AR-15 bolts.

We're also covering why there is no reason to fret over whether your bolt carrier group has a 9310 bolt, or a C158 bolt.

Grab a tissue and wipe away those angry tears, tier-one LARPers. Let's explain.

What is Carpenter 158 Steel?

Carpenter 158 is a case-hardened steel made by the Carpenter Technology Corporation, a company that, since 1889, has produced alloys for the defense and aerospace industries. C158 comprises mainly chrome and nickel.

Being case-hardened, C158 affords a wear-resistant surface, while maintaining a softer inner core. This affords the steel both good shock absorption and resistance to deformation when placed under high loads, and good resistance to scratching, marring, and pitting.

C158 is carburized (case-hardened by depositing carbon onto the steel's surface) at 1,550 to 1,650 degrees (F). It is then air cooled using inert gas before being reheated to 1,450 to 1,550 degrees (F). It is then quenched in oil to harden its core. Finally, the steel is tempered at 200 to 300 degrees (F) to relieve stress.

Once hardened, C158 yields approximately 1103 MPa (megapascals) tensile strength, with a yield point -- the point at which stress imparted on the steel causes it to permanently deform -- of 931 MPa.

What is AISI 9310 Steel?

The 9310 steel is similar in composition to C158. It, too, is a case-hardened alloy comprising mainly chrome and nickel, with the addition of molybdenum. Like C158, 9310 excels at being used to create high-wear components that are subjected to heavy loads.

9310 is carburized at 1,650 to 1,740 degrees (F), oil quenched, then reheated to 1,530 to 1,620 degrees (F). It is then quenched in oil a second time. Finally, the steel is tempered at 340 to 410 degrees (F) to relieve stress.

Once hardened, 9310 steel yields approximately 1,068 to 1,289 MPA tensile strength, with a yield point of 896 to 1,068 MPa.

9310 vs C158: Is One Better?

No. Let's address why neither steel (nor an AR-15 bolt made with either alloy) is better than the other. Let's directly address all the false claims perpetuated online about 9310 and C158.

9310 is not "7% stronger" than C158.

This is a common claim made online, and it's just not true. It is possible that, when heat-treated under specific circumstances, 9310 yields a slightly higher tensile strength. 

It's entirely possible the 9310 steel a manufacturer uses to make AR-15 bolts may, in fact, have an equal or lower tensile strength than a batch of C158 steel.

C158 is not "more brittle" than 9310.

Another claim about C158 that isn't true. We only need to look at the hardness ratings of C158 and 9310 to debunk this.

Tensile strength can be converted into hardness ratings across the three common scales: Vickers, Brinell HB, and Rockwell C. We can use a conversion calculator to find the hardness ratings of both C158 and 9310.

Converting C158's tensile strength, we find it has the following hardness ratings:

• Vickers: 332
• Brinell: 321
• Rockwell C: 33

Converting 9310's tensile strengths, we find it has the following hardness ratings:

• Vickers: 321 to 395
• Brinell: 311 to 375
• Rockwell C: 32 to 40

As the data show, 9310's tensile strength ranges allow it to have an equal, lower, or higher hardness rating than C158 (which directly translates into the layman's term "brittleness") depending on its heat treatment and resulting tensile strength.

C158 is not "more difficult" to heat-treat than 9310 (and vice versa).

Some "online experts" claim C158 is harder to heat-treat than 9310, or that the steel is more likely to be "incorrectly heat-treated." Others claim the same about 9310. None of these claims is true.

The carburizing, hardening, and tempering processes for both steels are nearly identical: Heat the steel, deposit carbon, cool the steel, heat a second time, quench in oil, and, lastly, temper. As importantly, steel foundries are multi-million-dollar operations ran by highly regulated, billion-dollar corporations. These corporations have, over decades, perfected these treatment processes. They are simply not in the business of producing inferior steels with subpar heat treatments. 

Both Carpenter 158 and AISI 9310 steels are made for other billion-dollar industries. These alloys are frequently used to build critical infrastructure and industrial equipment. Metal manufacturers are required to ensure their steels are properly fabricated before batches are sold to manufacturers.

When either steel is purchased from a reputable foundry, the AR-15 bolts made from that stock will have received the correct heat treatment.

Why Are Some Bolts Made From C158?

The answer has nothing to do with the performance of the steel compared to 9310. Colt Manufacturing, which makes the U.S. military's service rifles, has a longstanding contract with Carpenter Technology Corporation to use C158 in its bolts.

In other words, it comes down to simple cost. Colt has a lucrative and longstanding agreement with its steel manufacturer, and this steel has been tested and proven in combat for decades. 

As the adage goes, "if it ain't broke, don't fix it." So, Colt continues to use C158 bolts in their military rifles. And, because so many civilian AR-15 owners want "what the military uses," commercial bolt manufacturers continue making C158 bolts. The obsession with "mil-spec" AR-15 parts lives on.

Why Are Some Bolts Made From 9310?

Yet again, it comes down to simple cost. C158 is a proprietary steel produced only by Carpenter Technology, so they can charge a premium. Manufacturers must purchase large quantities of the stuff if they want to get their hands on it, which isn't always feasible for smaller companies that make individual AR-15 parts.

AISI 9310 is not a proprietary steel manufactured by just one company. Any steel foundry can manufacture it. This creates competition, lowering its cost and allowing it to be sold to part makers in smaller batches. So, it serves as an affordable, comparable alternative for commercial bolt and AR-15 part manufacturers who aren't in the business of fulfilling massive government contracts.

(Buy a Bolt From a Reputable Maker)

In the end, this is the correct answer to the "9310 vs. C158" question. It's not the steel that matters, but the manufacturer who makes the bolt from it. No matter the alloy it's made from, any AR-15 bolt made with properly machined lugs, a good protective coating, a properly staked gas carrier key, and quality ring seals on the bolt itself, will perform just fine, reliably cycling the thousands of rounds your AR-15 was designed to handle.

So, how can you determine whether a particular AR-15 bolt is reliable? Simple: Confirm the bolt's specifications, and read customer reviews. We stand behind our own bolt carrier groups, which have been purchased and ran by thousands of customers across our hundreds of AR-15 kits.

Speaking of coatings -- which BCG coating is best (and which ones are bad)? We compare them all here.

Need a quality bolt? Grab one of our BCGs here.