What is 4150 CMV? Mil-Spec Barrel Steel Explained

Gun barrels are all made from two types of steel: Stainless, and carbon (we cover the differences here). Most tactical rifle owners prefer carbon steel barrels for their hardness, heat resistance, and longevity. There are a few types of carbon steels used to make barrels, but one type of steel -- advertised as 4150 CMV -- is, by far, the most popular (it's what we use for most of our AR-15 build kits and uppers).

Why? It's the "mil-spec" steel that service-issued M4 and M16 barrels are made of. But there's a lot of misinformation, even some myths, surrounding this alloy. So, what, exactly is 4150 CMV and what isn't it? Here's the truth.

Types of Carbon Barrel Steels

To truly undestand this stuff, we need cover all the different carbon barrel steels. You'll need to know the differences to better understand the whole "4150 CMV" discussion:

4140 Carbon Steel

4140 steel is a type of 4000-series carbon steel that contains 0.80% to 1.10% chromium and 0.15% to 0.25% molybdenum. These two elements make the steel much stronger and harder than regular carbon steel. The "41" in the number identifies these two elements.

The "40" refers to the 0.40% carbon also added to the steel to make it even harder. This steel maintains a tensile strength of 655 mega-pascals, or 95,000 PSI. For reference, 5.56 cartridges are proofed 125% maximum pressure, or 77,958 PSI. This type of steel is the most affordable but least desirable.

4150 Carbon Steel

4150 steel is also 4000-series steel that contains chromium and molybdenum, like 4140. However, the "50" denotes that it contains 0.50% carbon, which is 0.10% more than 4140. The higher carbon content requires additional stress-relieving treatments.

This makes 4150 more expensive and harder than 4140. These barrels have a higher tensile strength of 730 mega-pascals, or 105,880 PSI.

Chrome-Moly Vanadium (CMV)

Chrome-Moly Vanadium, or simply "CMV", is the official mil-spec gun steel developed by Colt Firearms for the military-issued M16 and M4. It contains the same elements as 4150 steel, but it also contains Vanadium.

Vanadium is an alloy that significantly increases the strength, hardness, and high-temperature stability of the steel. This makes CMV one of the best steels for making machinegun barrels, which are subjected to fully automatic fire.

Wait, So What is 4150 CMV?

The label "4150 CMV" is used to advertise three types of carbon barrel steels under one name.

Those three types of steels include:

• Regular 4150 barrels
• Chome-Moly Vanadium barrels (CMV)
• 4150 barrels with Vanadium added (yes, this is different from "official" CMV)

Unless you're dumping a box magazine through an M249, the differences between these three steels are negligible and meaningless. For the civilian gun owner who fires semiautomatic weapons, like any AR-15, CMV provides no meaningful improvement in performance over 4150 by itself, or 4150 with Vanadium added.

So why is this even a thing?

Because tactical rifle owners tend to prefer -- if not obsess over -- "mil-spec" stuff, most AR-15 barrel and gun makers advertise their 4150 barrels as "4150 CMV." Some rifle makers (like FN and Colt) do, in fact, use "real" CMV in their civilian-market barrels (since these are the same barrels sold for government contracts), but most manufacturers just use good ole' 4150, with or without Vanadium added.

"But I want an official CMV barrel!" You say. Fret not, for there is a way to tell whether a barrel advertised as "4150 CMV" is, in fact, a mil-spec CMV barrel.

The MIL B-11595 Standard

Some CMV barrels are labeled "MIL B-11595." This is not a steel, but rather the official military specifications used to make proper mil-spec barrels. Some CMV barrels will be stamped or advertised with this "MIL" identifier. The MIL B-11595 includes the military's specification document that lists the metals and elements contained in "real mil-spec" barrels. 

Looking at the spec sheet, you can compare 4150 with Vanadium added to actual CMV. Both contain generally the same percentages of carbon, manganese, phosphorus, sulfur, silicon, and chromium. Mil-spec CMV might contain 0.05% more molybdenum, though this difference is, again, negligible. 

Again, this is why "4150 CMV" is a thing: It's convenient way to market three types of gun barrel steels -- which are technically different but almost identical -- under one not-so-confusing name, which satisfies the "mil-spec" requirement so many tactical rifle owners demand.

Your barrel's coating is just as important as its steel. Next, learn the differences between QPQ nitride and chrome lining.