A match-grade barrel is a big investment for many shooters,
so how much life you can expect from it can be a hot topic. This article will
focus on an aspect of barrel wear that doesn’t seem to get much attention, but
is especially important for shooters who compete precision rifle matches where
you may fire 100-200 rounds, or if you use factory ammo where the seating depth
isn’t customized to your specific chamber.
So, we all know that over time a barrel wears out. Barrels on a precision rifle are like tires on a car: while some may be better than others, none last forever. They all eventually get so worn that they need to be replaced. The wear happens gradually over time – it’s not like your tires looked brand new for the first 5 years, and then overnight they became unusable. Rifle barrels also erode gradually through use.
Over time the first couple of inches in a barrel that are just forward of the chamber will wear in a couple of ways: Cracks and pits will develop, and the lands/rifling will erode. (Note: I give a summary of why and how that happens, and what you can do to extend barrel life in this post.)
As the barrel starts to reach the end of its accurate barrel life you typically see two things: Muzzle velocity will start slowing down and/or groups may open up (i.e. precision starts to wane). In my experience, which of those happens first seems to depend on the cartridge you’re shooting and how many rounds you usually fire before allowing the barrel to cool. The whole concept of “accurate barrel life” is subjective and varies person-to-person. That’s why you see a huge range of round counts in forums when people talk about barrel life for a specific cartridge. What one shooter sees as acceptable precision or muzzle velocity can be different than another, which makes it hard to discuss barrel life in absolute terms. There are also so many factors that can affect how a barrel wears that a simple round count isn’t enough to make an apples-to-apples comparison.
In this article, I’ll focus on how quickly the lands of a rifle barrel usually erode (especially for mid-sized cartridges that are popular in precision rifle matches), and how some shooters manage those changes over the life of the barrel.
How Quickly Will the Lands
Short answer: “It depends.” But, I realize that isn’t helpful, so I’ll try to expand. First, it varies by cartridge, how hot of a load you’re running, and how hot you allow the chamber to get. If you fire long strings without stopping to let the chamber cool, it can erode much faster – maybe exponentially faster. In fact, it’s possible to burn out a barrel in one afternoon if you never let it cool (read article). It can even vary based on the barrel-to-barrel variation in steel, whether the barrel was button-rifled or cut-rifled, or the specific type of gun powder you use – so there are too many factors to make sweeping generalizations.
I have measured the barrel erosion on a few of my barrels (because I’m a nerd like that). On one of my 6mm Creedmoor barrels I used for PRS/NRL style matches, I measured the Cartridge Base To Ogive (CBTO) distance required to barely touch the lands on a brand-new barrel and then measured the same barrel using the same method after 1300 rounds (I documented round count, also because I’m a nerd like that). There was 0.077” of difference between the two measurements. That means the lands on my 6mm Creedmoor eroded 0.077” over 1300 rounds, which is an average of 0.0059” every 100 rounds (i.e. 0.077 ÷ 1300 × 100).
0.077” is a relatively small distance. To put it into context, a credit card is around 0.032” thick, so the amount of erosion we’re talking about over the life of a barrel is barely more than the thickness of two credit cards. Of course, I’ve heard of guys who kept shooting barrels until there was over an inch of rifling missing, but less than 1/10th of an inch is more common.
Since the 6 Creedmoor barrel that I measured the total erosion on over the life of the barrel, I’ve started using a new 6CM barrel, and I precisely measured the distance to the lands after the first 60 rounds (so when the barrel was fairly “broken in”) and remeasured at 203 total rounds, and between those two measurements this new barrel was averaging 0.00699″ per 100 rounds over the early life of the barrel. I’ll share more on that later in the article, but that means my 6mm Creedmoor barrels typically erode by 0.006-0.007″ per 100 rounds.
Note: For context of my shooting style (since so much of barrel wear depends on that), I’d say it’s extremely rare for me to ever fire more than 12 shots in a row before I allow the barrel to completely cool. I’d bet 80% of the time I fire 6-10 rounds and then allow the barrel to cool to near ambient temperature. I have two identical 6CM rifles so that when I practice, I can allow one cool while I use the other and swap back and forth so neither ever gets excessively hot. Around 60% of the ammo that goes through one of my 6CM barrels is Hornady factory match ammo, and the rest is handloads that are on the warmer side of the spectrum – but not way over pressure like some guys load.
I also measured the total erosion on one of my 6XC barrels that I shot out a few years ago and it was close to the same my 6 Creedmoor. My 6XC averaged very close to 0.005” of erosion every 100 rounds over the accurate life of the barrel. One of my shooting buddies also measured 0.005″ every 100 rounds over the life of his 6XC barrel.
from AccurateShooter.com talks about erosion on a 6.5-284 barrel, and the
author says, “Since I started out, I’ve chased the lands, moving out the
seating depth 0.086 inches (2.18 mm). I always seat to touch. … I documented
every round through the gun and got 2,300 over four years.” That means his 6.5-284
averaged 0.0037” every 100 rounds (i.e. 0.086 ÷ 2300 × 100).
I searched the internet to try to find others who’d listed
the throat erosion they’d experienced with different cartridges, but there
are 2 fundamental issues we should be aware when it comes to data like this
found in the wild:
- Many people don’t use a measurement method that is repeatable within +/- 0.002”. Honestly, I didn’t until recently. To learn how to measure this more precisely, watch these videos: Mark Gordon Method, Alex Wheeler Method.
- Many people don’t carefully document round count, and are simply ball-parking it (i.e. “about 1000 rounds” could actually be 800 or 1200).
Of course, either of those two things could skew the data, and
if they’re both off it could lead to some bad conclusions. Having said that, I
did find a
forum post where a couple of the guys who seemed very detailed provided data
they collected over multiple barrels:
- 6mm Dasher: Averaged 0.004″ per 100 rounds
- 6.5×47 Lapua: Average of 0.004” per 100 rounds
Have you measured your barrel erosion? Let’s create a little data repository by sharing what we’ve measured in the comments of this post! Please provide: cartridge, total measured erosion, total round count, and maybe share how confident you are in the data. Share your data in the comments!
Here is a summary of the numbers we have so far to work
|Cartridge||Avg. Erosion Per 100 Rounds|
The cartridges above are representative of popular precision rifle cartridges used in competitions. I’d expect that cartridges that are more mild for their caliber size, like the 308 Win, would erode slower than what is shown. Likewise, I’d expect magnums, like a 300 Norma Mag or even 7mm Rem Mag, to erode faster than what is shown here. Hot-rod cartridges like a 22-250 or 22 Creedmoor that are pushing a lot of powder down a smaller caliber bore will likely also erode measurably faster. One of my closest friends has a custom rifle chambered in 6.5x280AI (yes, a wildcat of a wildcat – he’s a velocity fein). That is an excessively overbore cartridge, and he told me by 1200 rounds the lands had eroded by almost 0.200″! He didn’t have exact numbers, but his erosion was likely around 0.015″ per 100 rounds. Unfortunately, I’m not aware of anywhere to find that kind of data for different cartridges, but I’m hoping you (my readers) will share any data you’ve collected in the comments.
The Deception of
Now, we should be careful to understand that the numbers
above are “on average” for every 100 rounds. I didn’t measure the exact erosion
every 100 rounds, and it may not be linear across the entire life of the
barrel. It could be faster over the first 100 rounds, and slower over the
last 25% of the barrel life – or vice versa.
Over the past couple of months, as I’ve become more interested in this topic, I started measuring the throat erosion on my rifles more carefully and more frequently. I recently measured the distance to the lands on a new 6mm Creedmoor barrel after the first 60 rounds and found the CBTO dimension to touch the lands to be exactly 2.1660” with Hornady 110gr A-Tip bullets. A few weeks later, the same barrel had 203 rounds down the tube, and I measured it to be exactly 2.1760”. I actually repeated the measuring process twice, because I thought it was so unlikely that it’d have eroded exactly 0.0100” over the last 143 rounds – but I ended up with the same exact number both times, which says something about how repeatable my measurement method is! The lands eroding 0.0100” over 143 rounds equates to an average of 0.00699 per 100 rounds (0.0100” ÷ 143 round count × 100 = 0.00699″ per 100). So, over those rounds early in the barrel’s life it was very close to 0.007 per 100 rounds (at least for that barrel), unlike my last barrel which averaged 0.006” per 100 rounds over the full life of the barrel. I will say my new barrel is a different brand, and I’m using a different powder and bullet design than I was on the old barrel.
I say all that just so we don’t see any of these “average
per 100 rounds” as the unquestionable truth for how a barrel will wear. Not
only may it vary from one person to another, but for one barrel to another – or
even vary over the life of the same barrel. More detailed measurements would
need to be collected at frequent increments to determine the exact pattern,
although the results would likely vary by the specific barrel, load, and other
factors. (Note: If anyone knows of good research studies related to this
that would be applicable, please share them with us in the comments.)
I tried to illustrate 4 examples in the chart below that all
“average 0.004 inch per 100 rounds over the life of a barrel,” but each varied in
how they got there. The only thing we know for sure, is what your specific
barrel does as you shoot it won’t perfectly match any of the examples. It would
be a very interesting to see future research with multiple data points through
the life of the barrel for several types of cartridges and barrels.
Chasing the Lands
So, if we know the lands erode as the barrel wears, how does that effect our bullet jump? (What is bullet jump?) As the lands erode, bullet jump will naturally increase by around 0.004-0.007″ every 100 rounds for these mid-sized cartridges.
Let me call to mind two quotes I shared in the last post:
So, we know that changing bullet jump by 0.002-0.005 inches can potentially have a dramatic affect precision. That is why many precision shooters who reload will extend their seating depth regularly so their bullets will be seated further out as their barrel wears, often called “chasing the lands.” For example, if you are shooting with the bullet 0.010” off the lands, and the lands erode 0.030”, you now need to seat the bullet out 0.030” further to maintain 0.010” of bullet jump. If you don’t adjust seating depth, your bullet would be jumping 0.040”, which would likely no longer provide optimal precision.
It is critical for everyone to understand this, so I’ll quickly run through an example to make sure we’ll all on the same page: Let’s say you found that 0.020” of bullet jump was optimal for one of your rifles, and on your brand-new barrel that meant the CBTO dimension for a loaded round measured 2.500” to achieve that 0.020” jump. Then after 200 rounds, let’s say the lands had eroded by 0.010”. That means we’d adjust the seating depth on our die so the CBTO of a loaded round is now 2.510”, and that should put your bullet back to that relative distance of 0.020” jump to the lands again. The lands will continue to recede over time, so this is something you must repeat regularly to maintain the same bullet jump.
Important: Adjusting your powder charge or seating depth will affect your chamber pressure, so, as always, you should be careful when changing a load and watch for signs of excessive pressure. The Sierra Reloading Manual says that adjusting seating depth to match your rifle’s throat/freebore and maximize accuracy “is fine, but bear in mind that deeper seating reduces the capacity of the case, which in turn raises pressures. Going the other way, seating a bullet out to the point that it actually jams into the rifling will also raise pressures.”
What Does This Mean For PRS/NRL Shooters?
Based on info above, we can assume many mid-size cartridges popular in precision rifle competitions average around 0.004-0.007” of erosion every 100 rounds. Think about what that means over a two-day match where you fire 200 rounds. By the last stage your bullet jump could be 0.008-0.014” more than it was on the first stage! If the experts are saying that changing bullet jump by 0.002-0.005 inches can have a “dramatic” impact on precision, what will 2-7 times that much do?
As we saw in the last post, the overwhelming majority of professionally published materials from trusted experts suggest for optimal precision you should seat the bullet into the lands or very, very close to the lands. If you are seating bullets very close to the lands for minimal jump, like 0.010″ off the lands, then by the last stage your jump may have doubled – or more!
Some of you may already be thinking that you could simply start loading your 200 rounds of match ammo with increasing COAL. We just need to seat the bullet out 0.001” more every 16.67 rounds, right? Stop! Before we add yet another layer of complexity, what if we’re coming at the problem the wrong way? I’m not asking you to accept some radical new approach, but what if we simply pause for a minute to explore that idea.
Knowing that we’ll be in matches that require 100-200 rounds, maybe our priorities should be different than other shooting disciplines. In some of those other types of competitions, like Benchrest, shooters are literally loading their ammo at the range as they’re shooting. Other competitions, like Extreme Long Range (ELR), are much lower round count, and shooters need less than 50 rounds to compete over one weekend.
What if instead of looking for the one exact bullet jump that provides 100% optimal precision, we instead looked for the bullet jump that is very forgiving AND still provided good precision over a wider range of jumps?
Now let’s think about the guys who aren’t reloading: If we’re trying to optimize a rifle chamber for a shooter who plans to simply shoot match-grade factory ammo, could we optimize the freebore of the chamber to be a bullet jump that would continue to provide good precision over a longer period of time as the barrel wore? It seems like they might have similar priorities.
What if absolute peak precision might not be the only priority we are trying to balance? I totally understand that in some shooting disciplines, like Benchrest, optimal precision is the absolute highest priority and nothing else is even a close second. But, what if we came at this from a different angle and tried to see if there was a way to balance priorities of both precision and a forgiving bullet jump?
The graphic below is a matrix visualization of what I’m trying to describe. The yellow dot labeled A is a load absolutely optimized for peak precision, but is very sensitive to changes in bullet jump. However, the green area labeled B represents something that has very good precision, but is much more forgiving in terms of bullet jump.
The very next post will share some interesting new research that has never been published and is focused on this exact topic: If we were looking for the most forgiving bullet jump that still provides good precision over a wide range of jumps, what would we find? Stay tuned!
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