In general, there are two types of barrels that are generally available for AR-15s. They include:
- Chrome Moly, which is usually chrome-lined
- Stainless Steel
There are also a couple of ways that the barrel rifling is created that make a difference to some people, but not others. The processes for rifling bores are:
- Cut Rifled
- Button Rifled
- Cold Hammer Forged
Chrome Moly – Chromium-Molybdenum Alloy
Please note that not all chrome moly barrels are chrome-lined. You will also see nitride/melonite treated barrels, which do not use a plating process, so they are generally more accurate than chrome-lined. For some cheap barrels, you will also find chrome moly that is neither chrome-lined nor nitrided.
Military Specifications (Mil Spec) barrels fall into this category. The barrel bores are chrome-lined to provide:
- Corrosion Resistance
- Ease of Cleaning
- Extended Barrel Life
Chrome lining is absolutely a great feature if you are slogging through bad weather and are heating your barrel up to ungodly temperatures with extended fire. The typical shooter will never wear out a chrome-lined barrel in their lifetime. In fact, the barrel will probably last a couple of generations, at a minimum. To wear out a chrome-lined barrel would mean that you are probably shooting every day and making great big piles of brass.
Cons of chrome-lined are pretty much all based on accuracy. The way the chrome-lined barrels are made is that they are rifled larger than the bullet size in order to allow room for the lining. The problem is that the technology for putting in the chrome lining is a bit limited and results in a high and low points in the lining that impact the accuracy of the bullet. The debate is how much of an impact there is, but there is an impact because of the non-uniformity of the plating.
Stainless steel is perfect. OK, that is obviously not true. However, when it comes to marksmanship contests such as the NRA High Power Long Range shooters and Civilian Marksmanship Program (CMP) shooters, you will see every one of them using a match grade stainless steel barrel.
These barrels are not chrome-lined and they wear out quicker than a chrome-lined barrel, but we need to be perfectly clear in noting that is highly unlikely anyone would wear one out in their lifetime. To wear out a stainless steel barrel would require high volume firing and rapid firing resulting in excessive heat on a regular basis.
Stainless steel barrels also tend to be heavier than chrome moly.
The process is also known as single point cut rifling, is the oldest process used for rifling. It was invented in the 1500’s. The process is pretty simple. Basically, the barrel is bored out, and then a tool is pulled through, and rotated, the barrel cutting the grooves into it. It takes many passes to cut the grooves a little at a time to strip out the steel. Cut rifled barrels tend to be very accurate and the process doesn’t stress the steel like most other processes. The main issue is that the process is time consuming and labor intensive.
This is probably the most commonly used method for rifling barrels.
Rifle buttoning uses a tool of hardened steel or titanium carbide that is a negative image of the inside of the rifling. The tool is pushed or pulled through a barrel blank while it is rotated. Unlike cutting tools, this tool is forced through the barrel and engraves the ridges. The process is a cold process and puts stress on the metal of the barrel. While some metal is removed by pushing/pulling the button through, it is not cut like the cut rifling process and results in some of the metal be compressed as well as cut.
Since the process puts a great deal of stress on the barrel, the stress is relieved by heating the barrel afterwards, and the compressed metal will snap back a bit. To compensate for the metal that was compressed returning to its desired density, the button actually run through is a bit larger than the desired rifling size.
The main disadvantage of using this process is that if the metal is not uniform, it can result in the rotations not being perfect and the depth not being uniform for the grooves.
Cold Hammer Forged
Hammer forging is just like it sounds. Hammers are used to pound the barrel into its ship. A barrel blank is used, and a mandrel is inserted into the blank, then many hammers are used to pound the blank around the mandrel and putting the grooves into the inside. A mandrel is a carbide bar that has the grooves for what the barrel should look like on the inside. The hammers hit the barrel multiple times and very fast and compact the metal around the mandrel.
Like in button rifling, the metal is stressed and needs to be stress relieved, which is done by heating the barrels and letting the metal snap back into shape, a bit, after being worked.
The result of this process is a very uniform and very accurate barrel.
The costs, as you can imagine, are pretty high for this process. The machinery is expensive.
Barrels are made many different ways using different materials, but it comes down to this:
- Chromoly is the cheapest and lightest
- Chrome-lined is the best for rapid fire, and lasts the longest, but it is less accurate
- Stainless steel is the most accurate, and when it is hammer forged, it is amazing stuff that costs more.