The Dreaded Carbon Ring | What Is The Truth?

A carbon ring, probably one of the most thrown around terms when it comes to reloading and precision rifle problems. The problem is, when asked, most shooters don’t really know exactly what it is, and hardly anybody exactly knows what causes it.

One thing is known for sure, in most cases it does affect precision and pressure. Sometimes it even increases pressure to a dangerous point, so knowing what causes it, in order to prevent it, is crucial for safety.

What causes a carbon ring?

What we know for sure, is that a carbon ring in a barrel, as the name implies, is caused by burned carbon deposits that pack on and harden at a specific area. It becomes very hard, often having been referred to as almost diamond-like, possibly because a diamond is also made from heated and hardened carbon, and because it is often very hard to remove once formed.

The cause of that carbon being deposited and packed on there however, could be caused by a number of reasons, from case neck length in relation to chamber all the way through to the freebore diameter of a bullet chamber. Although a carbon ring is in the barrel, it is more specifically usually found in the neck area of the cartridge chamber of the barrel.

Carbon Ring Location

The true actual “carbon ring” that is traditionally referred to and was coined by benchrest shooters, is at location (2) on the image above. That is where when not cleaned thoroughly, diamond-like hard carbon gets baked on after many rounds. Normal carbon build-up in the barrel is removed with conventional cleaning, but the hard baked-on carbon there does not completely get removed, and slowly over times builds up until it suddenly touches a bullet in the lands, or before the lands, then suddenly created strange jumps in pressure and random flyers.

Nowadays, when people say they have a carbon ring, especially with the modern calibers, they mostly talk about the carbon build-up in the neck area of the chamber, at location (3) on the image above. This is often caused by a few different reasons. One reason, is when the freebore diameter of the reamers used to make the chamber, is too tight, it doesn’t allow enough of the carbon to escape the chamber when firing rounds, and causes excessive early carbon build-up in that area. The second reason is that most traditional barrel cleaning techniques, focus on the barrel, and misses this area, as the brushes, patches and jags for the barrel, is too small diameters to reach this bigger neck area.

Neck Length

Neck length, so what does that have to do with a carbon ring? Well, a lot actually. Often the first sign of a carbon ring, is usually heavy bolt closure or heavy bolt lift. These can be caused by the neck pushing into or being pinched by the carbon ring in the chamber neck area.

Often if necks of cases are trimmed too short, there is a bigger gap between the case mouth and the end of the neck space in the chamber, and that gap is where there is more space for carbon to accumulate.

When a carbon ring accumulates in that area too much, even necks trimmed short enough, can touch or push into that hard carbon ring when chambering a round, which could then cause the heavy bolt closure. What can also happen, is when a shot is fired, a case expands and the neck is jammed into the hard carbon ring, then heavy bolt lift could occur. (Note: heavy bolt lift could also be caused by a carbon ring, when it increases the pressure to the point where the case slams into the bolt face and then gets stuck on the extractor or ejector pin.)

If a neck is not trimmed and stretches to a point where the case itself is longer than the chamber, then even without a carbon ring, the case mouth with press into the neck area shoulder in the chamber, and that could cause heavy bolt closure and heavy bolt lift after the shot. Generally, older cartridge designs that have shoulder angles of less than 30 degrees, stretch more with firing than newer designs with steeper shoulder angles.

Freebore Diameter

What is often blamed on slightly short necks, or “dirty” powder causing carbon rings, is actually caused rather by a freebore diameter being too tight. This not being so common on factory chambered rifles which were chambered with SAAMI spec reamers, but more often with wildcat cartridges, or custom reamer cartridges.

We see this more often among newer 6mm cartridge design shooters like the 6mm Dasher and 6mm GT. They get a new rifle built, then often quickly find they get heavy bolt lift after less than 100 shots after cleaning or increased pressure spikes. This even with lower than maximum charges. Usually when looking for advice, the first suggestions they get and the first things that are blamed, is brass, or “dirty” burning powder.

What I and a few gunsmiths and high-regarded shooters I know have found, is that the majority of the time, that carbon ring forming prematurely, is almost always caused rather, but a small freebore diameter. This then blocks burning carbon and gases from passing the projectile in the chamber and barrel while firing, and then sends them back into the chamber to go and stick and build up wherever they find space. This space is usually then that space between the case mouth and the chamber shoulder step.

With many modern 6mm cartridges like the 6mm GT and 6mm Creedmoor, the published SAAMI chamber dimensions show a .2436″ or .2435″ chamber freebore diameter. This is a bad design from this aspect, yes, even the esteemed GA Precision gets it wrong, which is proven by the number of carbon ring and carbon build up complaints on the 6mm GT Facebook group. One user even complaining: “

Although no SAAMI specs exists for it, the 6mm Dasher reamers also mostly have this same .2436″ freebore diameter. Usually when shooters of these calibers complain of premature carbon buildup or carbon ring in chamber, we find their chamber was turned with a reamer that has a .2436″ or smaller freebore diameter. So all their brass trimming and other options does not fix it, especially with a 6mm Dasher where the brass most often never stretches enough to touch the shoulder step in the chamber. In almost all these cases, we put another reamer through opening that chamber freebore to .2438″ or .2440″, we find the problem solved right away.

So, if you are planning to build or re-chamber a rifle in 6mm Dasher, 6mm Creedmoor, 6mm GT or any other modern 6mm cartridge, get the chamber cut with a reamer that has a freebore diameter of .2438″ or larger.

Carbon Ring Causing Pressure

How does a carbon ring cause an increase in pressure? If the carbon ring build-up becomes enough, it can constrict the bullet or case neck, thus causing the pressure of the shot to rise. The carbon decreases chamber size, also increasing pressure. Carbon in the lands can decrease bullet jump or cause the bullet to jam, also causing a change in pressure.

Cleaning Out A Carbon Ring

Carbon rings, because of the nature of what they are, hardened carbon, and being able to become harder than steel, can be really difficult to remove. Normal cleaning, or even abrasive sometimes is not sufficient to remove it, even to the point where you can damage the inside of your barrel chamber or bore, and still not remove the carbon ring.

It is recommended to use a cleaning approach combining chemical and abrasive removal, so a carbon solvent together with physically scrubbing the softened carbon with a copper or nylon brush.

Neck Area Carbon Ring Cleaning

So I personally generally like to have a brush with a patch soaked in a carbon solvent pushed into the carbon ring, and then leaving it there to soak and penetrate the carbon for 15-20 minutes. I like to use something like CLR or Boretech Carbon Remover. Just be careful with CLR, as it can be corrosive to steel and other materials and finishes, so do not let it touch other surfaces like the trigger, action or stock, and do not let it soak for more than 20 minutes.

Thereafter, I like to spin the brush in that neck chamber area to scrub the softened carbon out. This can be done with either a nylon or copper brush, but the brush needs to be large enough in diameter to touch the carbon rings in the shoulder area. So for instance if you have a 6mm caliber, the neck chamber diameter will usually be around .274″, so a .277″ (.270 caliber) or .30 caliber brush would clean that area well. With a copper brush, turning the rod by hand is usually enough, but with a nylon brush, I find it only works on a hard carbon ring if I spin it with a cordless drill to scrub that area.

False Carbon Ring Caused By Shadow

Very often, guys who have just recently got their first borescope, think they have a carbon ring when inspecting their bore and chamber for the first time. I have seen it countless times on social media and online forums. But what most of the inexperienced users first think is a carbon ring, is merely the shadow cast by the light of the borescope over the step of the barrel chamber neck shoulder, with the borescope inserted into the barrel from the muzzle. If they then insert the borescope from the action side after instructed, the light comes from the other side, and this shadow is clearly gone and no dark ring or carbon ring is visible at all.