With the number of “overbore” rifle cartridges popular today, barrel erosion is becoming a hot topic. For example, competitive shooters using a 6 PPC typically replace a barrel after 700-800 rounds, and a 6.5-284 might also need to be replaced before the 1,000 shot mark. That means the cost to keep a good barrel on the gun could be 60-70¢ per shot before we even consider the cost of components like match-grade bullets, brass, primers and powder. In reality, short barrel life could easily double your cost per shot. So what can we do to prolong the accurate life of our barrels?
Understanding Barrel Erosion
When people talk about a barrel being “shot out” they typically are referring to the wear that occurs just forward of the chamber in the first 4 inches of the rifling known as the throat. This is a natural phenomenon that occurs in all barrels given enough time, and is due to many factors that will be outlined below. This barrel wear could be manifested in a few different ways:
- Severe wear to the lands and grooves (likely only a few thousandths to a hundredth of an inch)
- Cracks and pits in the barrel (these were likely imperfections present in the barrel during the time of manufacturing that “grow” over time under the intense heat and pressure)
The result isn’t typically a catastrophic failure, but simply a slow decline in accuracy, which makes pinpointing when this has occurred very subjective. There are a couple ways to have a more objective approach to determine if a barrel has “worn out”:
- Keep detailed records of the rifle’s performance so you know its level of accuracy and can recognize a decline in accuracy when it occurs
- Inspect the barrel with a borescope (Hawkeye is a trusted brand, and can be found from Sinclair for $700-1000)
- Pay a gunsmith to check the barrel
What Causes It?
There are a number of factors that play into barrel erosion, and these have traditionally been broken down into three categories. However, these factors don’t function independently of each other, but are tightly coupled and act in concert together in an interdependent fashion.
- Thermal – Include bore surface phase changes (i.e. transitioning between solid, liquid, gas phases), softening and melting, as well as cracking due to expansion and contraction associated with the barrel heating and cooling.
- Chemical – Include carburizing or oxidizing reactions, which are chemical processes that occur at the bore surface under extreme heat. These cause the barrel to change at a molecular level.
- Mechanical – Includes erosion caused by direct impingement of gas and solid particles traveling across the bore surface.
For a complete technical explanation of barrel erosion, see this white paper published by the Australian Department of Defense: Understanding and Predicting Gun Barrel Erosion by Ian Johnston.
What Can We Do?
We can take strategic steps to combat each category of barrel erosion:
- Reducing heat and/or pressure inside chamber and barrel
- Modify cleaning methods
- Refurbish or “set back” barrel
- Chase the lands
- Upgrade to premium barrel
Reduce Heat and/or Pressure
- Reduce Load – This may not be an option for velocity-fiens, but backing off from the max load can significantly cut down on chamber pressure and greatly extend your barrel life. A recent study by the U.S. military shows an almost linear relationship between the amount of powder used and barrel friction.
- Slow shooting pace – If you are able to allow your barrel time to cool between shots you can drastically reduce the maximum temperature reached in the throat and barrel. Some shooters switch out rifles or barrels when the barrel starts to heat up and they need to shoot extended strings. To understand how much this can impact barrel life, reference this study: How to Wear Out a Barrel in One Afternoon
- Coat Bullets– Although a controversial in shooting sports, many competition shooters claim to get almost double the barrel life when coating bullets. I believe a study recently conducted by Norma proved that coated bullets do improve barrel life (results shown in the diagram to the right). There are three common types of bullet coatings:
- Molybdenum Disulfide (Moly)
- Tungsten Disulfide (WS2 or Danzac)
- Hexagonal Boron Nitride (HBN or “White Graphite”, used by David Tubb)
David Tubb on bullet coating: “I have found that moly coated bullets provide extended barrel life. I believe that the coating itself provides a “buffer” of sorts between the powder gases and barrel surface, and also that moly coated bullets result in less heat being transmitted to the barrel. Many time Bianchi Cup champion, Doug Koenig, told me that he can make several more practice runs firing the Barricade Event using moly coated bullets. In this event the shooter secures the handgun barrel against the barricade using his hand; barrel heat build up dictates how long the shooter can make practice runs. Again, moly coating reduced heat build up sufficiently that Doug could get in 3-5 more 6-shot strings before having to stop and allow the barrel to cool. It’s been my experience that moly coating adds at least 20 percent to accurate barrel life. Barrel wear in a centerfire rifle is almost exclusively due to throat erosion (cracks and roughness in the first 3-4 inches ahead of the chamber caused by heat, flame, and pressure). An additional 500-plus rounds may not seem like much, but it will add up over the course of a few barrels. However, that, like many advantages of moly coated bullets, are welcome side benefits to the major improvements that result from their use.” (Update: David Tubb now believes the benefits of boron nitride coating far exceed that of moly).
- Change Powder – Changing to a different powder may reduce both heat and pressure. Typically powders with slower burn rates reduce throat wear.
- Treat/Condintion Barrel – There are a few different methods of treating a barrel that claim to extend barrel life, primarily by reducing the friction in the barrel and/or hardening the surface of the barrel.
- HEF USA offers the patented Melonite process for barrels for around $100. This provides a thin (10-20 micron) layer of surface hardening. This produces a very slick, very hard surface that provides reduced friction in the bore. Early tests show Melonite barrel treatment can reduce throat wear and increase useful barrel life.
- David Tubb’s FinalFinish Throat Maintenance Systems (TMS) is a “bore conditioning system” containing 75 bullets impregnated with different polishing compounds. – According to David Tubb: “TMS greatly extends accurate barrel life. These bullets are coated with a specially chosen polishing compound that will smooth and maintain the throat in your firearm, thereby significantly offsetting the effects of throat erosion – the number-one culprit in barrel wear and accuracy loss. FinalFinish TMS bullets are specially engineered to smooth and polish this eroded area, helping restore and maintain smoothness. Using the TMS bullets as prescribed should extend the life of a barrel by at least one third. For instance, the .308 barrel that normally gives 4000 accurate rounds should now easily provide 5500 accurate rounds. The .22-250 that was shot out before at 2000 rounds should be on the money for at least 3000 rounds.”
Modify Cleaning Methods
Many barrels have been prematurely worn out from improper cleaning techniques. Here are a few cleaning tips:
- Cleaning is not a completely benign procedure so it should be done carefully and no more than necessary.
- Always clean from the breech whenever possible, pushing the patch up to the muzzle and then back without completely exiting the muzzle. If you exit the muzzle, the rod is going to touch the bore and be dragged back in across the crown followed by the patch or brush. Try to avoid dragging items in and out of the muzzle, it will eventually cause uneven wear of the crown.
- Use quality, name brand chemical solvents (e.g. Butch’s Bore Shine) that reduce the need for aggressive brushing
- Don’t use abrasive cleaners. There are essentially two types of cleaners: chemical and abrasives. The chemical cleaners are usually a blend of various ingredients including oils, solvents, and ammonia (in copper solvents). The abrasive cleaners generally contain no chemical solvents and are an oil, wax, or grease base with an extremely fine abrasive such as chalk, clay, or gypsum.
- Use a quality, one piece carbon fiber or coated cleaning rod with a freely rotating handle.
- Always use a bore/rod guide that fits both your receiver raceway and the rod snugly. How straight and how snug? The object is to make sure the rod cannot touch the bore.
- If you choose to use brushes in your cleaning process, use softer nylon brushes
- Avoid ammonia – Some copper solvents contain a high % of ammonia. This makes them a great copper solvent, but if left in the bore too long, can damage/corrode the steel. Do not leave these chemicals in a bore any longer than 10-15 minutes MAXIMUM! DO NOT EVER use straight ammonia to clean a barrel.
People often mistake a barrel for being “shot out” that might just need a tune-up. Before you replace your barrel, consider these options:
- Recrown – Although erosion at the throat is more common, there can also be wear or damage at the crown of the barrel. According to Krieger Barrels “even slight damage to the barrel crown is extremely detrimental to accuracy.” Crown damage can sometimes be mistaken for a barrel being “shot out”, so paying a gunsmith to check and recrown the barrel can often times bring accuracy back for the fraction of the cost.
- Set-Back Barrel – This involves a gunsmith cutting off around 1” of the barrel (on the chamber side), then re-reaming the chamber. This essentially removes the part of the throat just forward of the chamber, which is where primary throat erosion occurs. You may lose an inch or more of length in the set-back process, but can dramatically extend barrel life. When purchasing a barrel for an “overbore” cartridge, I buy one that is 1” longer than what I really want so I can set it back when the throat starts to erode and still have plenty of barrel length to get the velocities I’m targeting.
Chase the Lands
As the lands erode in the throat of the rifle, one option is to just start increasing the seating depth of the bullet so it maintains that relative position to the start of the rifling. This may not be possible if you are using a magazine fed rifle and the cartridge overall length is already near the max allowed in the magazine. That is why ensuring you have some extra room in the magazine is a key point to consider when selecting a cartridge, but is often overlooked until it becomes a problem.
Upgrade to Premium Barrel
Most agree that a premium barrel (e.g. Krieger Barrels, Bartlein Barrels, etc) will generally have a longer accurate lifespan than a stock, factory barrel due to their superior quality. To see the dramatic difference between these two types of barrels for yourself, check out Lilja’s borescope video showing both a brand new factory barrel and a custom, hand-lapped Lilja match barrel: