Carbon Steel Barrels
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 megapascals, 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.
4150 steel is also 4000-series steel that contains chromium and molybdenum like 4140 steel. However, the "50" denotes that it contains 0.50% carbon, which is 0.10% more than 4140 steel. The higher carbon content requires additional stress-relieving treatments. This makes 4150 more expensive and harder than 4140. It also produces a higher tensile strength of 730 megapascals, or 105,880 PSI.
CMV is the most popular type of steel you'll see advertised when
shopping for an AR-15 barrel or upper assembly. The problem is, this
label is a misnomer. The phrase "4150 CMV" is a term barrel makers came
up with decades ago that sticks around to this day 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. It also contains Vanadium. Vanadium
significantly increases the strength, hardness, and high-temperature
stability of the steel. This makes CMV the most suitable steel for
producing machinegun barrels for full-auto or rapid fire.
Stainless barrels don't suffer the same confusion as carbon steel AR barrels. There are four "tiers" of stainless steel barrels: 410, 416, 416R, and 17-4 PH.
Advantages of Stainless vs. Carbon Steel
There are two major advantages to picking a stainless barrel over a carbon steel barrel: First, it'll provide better corrosion resistance with better accuracy. A carbon-steel barrel could last longer if it has a chrome-lined finish, but chrome-lining affects rifling and shot placement. Second, a stainless barrel will provide better "toughness". Stainless barrels are more resistant to heat and abrasion than carbon steel. All other factors being perfectly equal, a stainless barrel will last longer than a carbon steel barrel.
Barrels made from 410
stainless steel include the element molybdenum. The inclusion of
molybdenum is meant to reduce the risk of developing sulphide stringers.
This stainless steel is actually more durable than 416 and 416R
stainless, offering the longest barrel life of the three. The only
drawback to 410 stainless is its ability to withstand cold temperatures.
Firing rounds through a 410 stainless barrel while below freezing
temperatures (32 degrees F) increases the risk of cracking the barrel or
causing a catastrophic failure due to sulphide stringers in the metal.
416 stainless boasts the highest machinability of any stainless steel. This stainless can be easily cut like a carbon steel barrel, allowing for better rifling and more consistent performance. Unfortunately, 416 stainless is so "workable" because it contains more sulphur than 410 stainless. This higher inclusion of sulphur greatly reduces the corrosion resistance, weldability, and formability when compared to 410 stainless. (We recommend avoiding purchasing a typical 416 stainless barrel.)
416R stainless is a newer, proprietary stainless alloy produced by Crucible Industries, an industrial metal supply company. 416R boasts high machinability (and thus accuracy) like regular 416 stainless, except it also includes molybdenum (like 410 stainless). It also contains less sulphur than 416, reducing the risk of catastrophic failure from the inclusion of sulphide stringers. Crucible confirms an AR-15 barrel made from 416R stainless can be safely fired at temperatures as low as -40 degrees (F).
shooters laud the advantages of 17-4 PH stainless over the more typical
416R and 410 stainless options. The "PH" in 17-4 stainless stands for
precipitatino hardening. This process involves using heat to make the
alloy stronger by hardening it and introducing precipitates (fine, solid
impurities like magnesium, aluminum, titanium, and nickel) to the
steel. This process also reduces deformations and warping, producing an
incredibly honed barrel.
17-4 PH also contains 17% chromium, lending a natural hardness, resistance to heat, and smoothness to the rifling and bore. 17-4 PH barrels do not suffer the same cold-weather concerns as 400-series barrels. On the opposite extreme, the inclusion of precipitates also allows the barrel to withstand extremely high temperatures without deformation, meaning shooters can stay on the trigger and put more shots down-range without the barrel expanding and loosing accuracy. 17-4 PH barrels are usually hardened to around 36 to 44 RC. 416R barrels only harden to 28 RC.
The only downside to 17-4 PH barrels are their ability to withstand sustained, rapid fire. Once the metal is heated nearly to its tempering point through sustained fire, it becomes permanently soft and the barrel is no longer capable of performing accurately (or safely). But getting 17-4 PH to such temperatures is difficult and would take hundreds (if not thousands) of rounds. A 17-4 PH barrel is superior in every way, offering sub-MOA accuracy out to hundreds of meters. They typically cost 3 to 5 times more than a 416R barrel.