Throat Erosion Rates

[222Jim]

Adjusting bullet jump is an accepted way to improve precision. So much so that there’s a lot of advice on what jump (or jam) works best and how much you should adjust the seating depth by to find the perfect jump. But, throats erode, and if jump is so important, shouldn’t throat erosion as well? Plus, if I knew how fast throats erode, wouldn’t I be in a better position to plan how to get the most out of my barrels before they “die”? Sadly, there’s actually little quantitative information on how fast throats erode. So, I decided to determine erosion rates using my .222 Remington, .22-250 Remington, and .308 Winchester.

While working on this project of mine I was able to put numbers against things I knew. For example, the throat erosion rate of my (overbore) .22-250 is four to five times faster than that of my (tame) .222.

But I was also surprised by the fact that using different bullets to determine erosion rates gave me significantly different (and statistically valid) results. What I figured out was it’s the ogive/leade “curve/inclined-plain” geometry that creates an illusion that a longer radius ogive means more erosion:

• In reality, as the leade erodes to a shallower angle, a bullet with a longer tangent radius ogive moves incrementally further, relative to shorter radius bullets, before contacting the eroded leade.
• On the other hand, secant bullets “hide” erosion given they, with their sharp bearing-surface/ogive transition, won’t move further into the throat until the leading edge of the leade has been eroded down to the bore diameter.

In summary, typical erosion rates I determined using different bullets in my .222 and .22-250 are:

Inches/100-Rounds​	Berger FB Varmint​	Hornady V-Max®​	Nosler Varmageddon®​	Sierra BlitzKing®​
​	40, 52, 55 & 60​	40, 50, 53, 55, 60​	40, 50, 53 & 55​	40 & 50
.222 Remington	​	​	​	​
0 to 1,045 Rounds​	0.0009 – 0.0010​	0.0004 – 0.0005​	0.0009​	0.0004​
1,046 to 1,850 Rounds​	0.0017 – 0.0021​	0.0011 – 0.0017​	0.0016 – 0.0020​	0.0015 – 0.0016​
.22-250 Remington	​	​	​	​
0 to 1,113 Rounds​	0.0037 – 0.0040​	0.0023 – 0.0028​	0.0037 – 0.0045​	0.0031​

I only have results using four bullets for my .308 after 484 total rounds, and those erosion rates are comparable to my .222 at between 0.0005” to 0.0014”/100-rounds.

So, how will knowing all of this change how I plan to use by rifles?

• The barrel of a .22-250 is reputed to have a life of ~3,500 rounds, and that happens to coincide with my own experience. The throat will also erode by ~0.140” over that lifetime. So, I won’t tinker with 0.020” jump to jammed by 0.005”. Instead, I’ll stick to one specific bullet that meets my precision goal with lots of jump. For my .22-250, that’s the Berger 40 gr FB Varmint seated with over 0.100” of jump.
• The barrel of a .222 is reputed to have a life of ~7,500, and the throat will erode by ~0.140” over that lifetime. That’s a long life, so my load development will be broken into a middle age, when I’ll still tinker with 0.020” jump to jammed by 0.005”, and old age, when I’ll settle on a jump tolerant bullet. When does this rifle enter old age? When I can’t keep 1 caliber (0.224”) of bullet in the case neck.
• My .308? Based on what I know right now, I’d say that what’s good for my .222 is good for my .308.

If this has interested you and you’d like to learn more about what I found, including more detailed information on erosion rates vs. different bullets, as well as some statistics and analysis, then please open the attached pdf where I get into more detail on what I found, how I found it, and so on.

Finally, I’d love to receive your feedback, thoughts and insights.

 

[Murray Brook]

Interesting. I have a 222 and have had 2 different 22/250's. Never checked the Rem 222 much but both 22/250's I had went south at appx 1700 rounds ( .010 jump using 52 Bergers)

 

[222Jim]

Interesting. I have a 222 and have had 2 different 22/250's. Never checked the Rem 222 much but both 22/250's I had went south at appx 1700 rounds ( .010 jump using 52 Bergers)

Assuming your .22-250 erodes as fast as mine, and if you set that 0.010" jump "new", then it grew by ~0.060" to have 0.070" of jump............

 

[powderbrake]

Yours is an excellent analysis of the erosion, thanks for the info.

My question is, "do you measure and adjust the jump on your 22-250, or simply seat all your 40 gr FB Varmint Bergers at a fixed setting throughout the life of the barrel, and expect the group size to be constant"? Maybe seating is fixed and you expect the group size to remain "acceptable".

Your new barrel data shows the group size to be .16" mean radius (which I convert to .32" center to center in my mind), this is very good, but I suspect that it will grow with barrel age. What is your "acceptable" group size with no changing of the seating depth. I guess this is your "precision goal", but I don't want to put words into your mouth.

 

[222Jim]

Yours is an excellent analysis of the erosion, thanks for the info.

My question is, "do you measure and adjust the jump on your 22-250, or simply seat all your 40 gr FB Varmint Bergers at a fixed setting throughout the life of the barrel, and expect the group size to be constant"? Maybe seating is fixed and you expect the group size to remain "acceptable".

Your new barrel data shows the group size to be .16" mean radius (which I convert to .32" center to center in my mind), this is very good, but I suspect that it will grow with barrel age. What is your "acceptable" group size with no changing of the seating depth. I guess this is your "precision goal", but I don't want to put words into your mouth.

Thanks for the feedback.

With respect to seating depths in my .22-250 (1:14 twist by the way), I now only shoot 40 gr bullets, and I seat all (Berger FB Varmint, Hornady V-Max, Nosler Varmageddon and Sierra BlitzKing) at what I originally measured as 0.120" jump (maximum jump with ogive at case mouth 0.160"). That, in this little bullet, just allows the bearing surface to slightly protrude from the neck. I could get as close as jamming, but I had less than 0.030" of bearing surface inside the case neck. Of course, your rifle and your cases might allow more or less.

My minimum acceptable group size is a mean radius of 0.25", equivalent to a CEP 90% of 0.42. How did I arrive at that? One of the best pieces of advice I ever read with respect to how not to chase your tail forever, is to decide "what's accurate enough", and once you get there, stop. The smallest critter I shoot are our local "gophers", and I want to hit them with a clean kill at 300 meters 90% of the time. After figuring out how big a gopher (Richardson's Ground Squirrel) is, I did some math to convert that into a mean radius and CEP. I have met that goal with all of those bullets except Nosler Varmageddon (a frustratingly close but not close enough 0.26 mean radius). But, I am so darn close I'm using up my inventory of those bullets on them anyway.

Coyotes are bigger, so if it works for a gopher, it'll work for them too (although I tend to limit myself to 200 meters on them purely to make sure the bullet has enough "wack", a.k.a. terminal energy, to kill them cleanly.

 

[Murray Brook]

My question is, "do you measure and adjust the jump on your 22-250, or simply seat all your 40 gr FB Varmint Bergers at a fixed setting throughout the life of the barrel, and expect the group size to be constant"? Maybe seating is fixed and you expect the group size to remain "acceptable

I'll give you my experience on this. My first 22/250 was a 14 twist Remington. My CBTO remained the same through out the accurate life of the barrel. My other 22/250 was a 14 twist Sako A11. To maintain good accuracy I had to "chase the lands". When I couldn't get enough of the bullet in the case to make things work I rebarreled to a 6mm BR Norma. JME

 

[222Jim]

I'll give you my experience on this. My first 22/250 was a 14 twist Remington. My CBTO remained the same through out the accurate life of the barrel. My other 22/250 was a 14 twist Sako A11. To maintain good accuracy I had to "chase the lands". When I couldn't get enough of the bullet in the case to make things work I rebarreled to a 6mm BR Norma. JME

Rebarreled to 6mm BR Norma. Interesting. I've heard of people having the chamber end cut down by 1 or 1-1/2 inches and then rechambered in the same caliber. How's the 6mm BR?

 

[Murray Brook]

Cutting the chamber and redoing with the same barrel is not very cost effective. The rechambering will only last- maybe- 500 more rounds. If you can do this yourself then yes if is worth doing, but having to pay to have it done is not a great idea. Going to a new barrel is what was recommended to me.

 

[Webster]

Adjusting bullet jump is an accepted way to improve precision. So much so that there’s a lot of advice on what jump (or jam) works best and how much you should adjust the seating depth by to find the perfect jump. But, throats erode, and if jump is so important, shouldn’t throat erosion as well? Plus, if I knew how fast throats erode, wouldn’t I be in a better position to plan how to get the most out of my barrels before they “die”? Sadly, there’s actually little quantitative information on how fast throats erode. So, I decided to determine erosion rates using my .222 Remington, .22-250 Remington, and .308 Winchester.

While working on this project of mine I was able to put numbers against things I knew. For example, the throat erosion rate of my (overbore) .22-250 is four to five times faster than that of my (tame) .222.

But I was also surprised by the fact that using different bullets to determine erosion rates gave me significantly different (and statistically valid) results. What I figured out was it’s the ogive/leade “curve/inclined-plain” geometry that creates an illusion that a longer radius ogive means more erosion:

• In reality, as the leade erodes to a shallower angle, a bullet with a longer tangent radius ogive moves incrementally further, relative to shorter radius bullets, before contacting the eroded leade.
• On the other hand, secant bullets “hide” erosion given they, with their sharp bearing-surface/ogive transition, won’t move further into the throat until the leading edge of the leade has been eroded down to the bore diameter.

In summary, typical erosion rates I determined using different bullets in my .222 and .22-250 are:

Inches/100-Rounds​	Berger FB Varmint​	Hornady V-Max®​	Nosler Varmageddon®​	Sierra BlitzKing®​
​	40, 52, 55 & 60​	40, 50, 53, 55, 60​	40, 50, 53 & 55​	40 & 50
.222 Remington	​	​	​	​
0 to 1,045 Rounds​	0.0009 – 0.0010​	0.0004 – 0.0005​	0.0009​	0.0004​
1,046 to 1,850 Rounds​	0.0017 – 0.0021​	0.0011 – 0.0017​	0.0016 – 0.0020​	0.0015 – 0.0016​
.22-250 Remington	​	​	​	​
0 to 1,113 Rounds​	0.0037 – 0.0040​	0.0023 – 0.0028​	0.0037 – 0.0045​	0.0031​

I only have results using four bullets for my .308 after 484 total rounds, and those erosion rates are comparable to my .222 at between 0.0005” to 0.0014”/100-rounds.

So, how will knowing all of this change how I plan to use by rifles?

• The barrel of a .22-250 is reputed to have a life of ~3,500 rounds, and that happens to coincide with my own experience. The throat will also erode by ~0.140” over that lifetime. So, I won’t tinker with 0.020” jump to jammed by 0.005”. Instead, I’ll stick to one specific bullet that meets my precision goal with lots of jump. For my .22-250, that’s the Berger 40 gr FB Varmint seated with over 0.100” of jump.
• The barrel of a .222 is reputed to have a life of ~7,500, and the throat will erode by ~0.140” over that lifetime. That’s a long life, so my load development will be broken into a middle age, when I’ll still tinker with 0.020” jump to jammed by 0.005”, and old age, when I’ll settle on a jump tolerant bullet. When does this rifle enter old age? When I can’t keep 1 caliber (0.224”) of bullet in the case neck.
• My .308? Based on what I know right now, I’d say that what’s good for my .222 is good for my .308.

If this has interested you and you’d like to learn more about what I found, including more detailed information on erosion rates vs. different bullets, as well as some statistics and analysis, then please open the attached pdf where I get into more detail on what I found, how I found it, and so on.

Finally, I’d love to receive your feedback, thoughts and insights.

Don't understand why your concerned only with throat erosion and ignoring barrel erosion. I cross-sectioned my 6mm Rem barrel and examined it on a scanning electron microscope. I never counted the rounds down the barrel. Probably 4000. Erosion was found at least 4 inches down the barrel. There was no rifling for at least 2".

 

[fkimble@charter.net]

First time I've heard of having the entire barrel redone into a new caliber. Creative approach.

I don't think that is what he did! He said rebarreled not rebored!

Frank

 

[jelenko]

Don't understand why your concerned only with throat erosion and ignoring barrel erosion. I cross-sectioned my 6mm Rem barrel and examined it on a scanning electron microscope. I never counted the rounds down the barrel. Probably 4000. Erosion was found at least 4 inches down the barrel. There was no rifling for at least 2".

Wow.
My 223 barrels regularly are replaced in the mid to high 3K round range. With a bore scope, I can't tell there's been any reduction in the lands. Of course, distance to the lands [mesured with the same 'reference' bullet over the past 10 yearserodes around 2-3/thousandths per 100 shots.

 

[Murray Brook]

How's the 6mm BR?

Great cartridge. Accurate and effective. Very easy to tune. Much greater barrel life and this is what I was after JMO

 

[powderbrake]

Thanks for the feedback.

With respect to seating depths in my .22-250 (1:14 twist by the way), I now only shoot 40 gr bullets, and I seat all (Berger FB Varmint, Hornady V-Max, Nosler Varmageddon and Sierra BlitzKing) at what I originally measured as 0.120" jump (maximum jump with ogive at case mouth 0.160"). That, in this little bullet, just allows the bearing surface to slightly protrude from the neck. I could get as close as jamming, but I had less than 0.030" of bearing surface inside the case neck. Of course, your rifle and your cases might allow more or less.

My minimum acceptable group size is a mean radius of 0.25", equivalent to a CEP 90% of 0.42. How did I arrive at that? One of the best pieces of advice I ever read with respect to how not to chase your tail forever, is to decide "what's accurate enough", and once you get there, stop. The smallest critter I shoot are our local "gophers", and I want to hit them with a clean kill at 300 meters 90% of the time. After figuring out how big a gopher (Richardson's Ground Squirrel) is, I did some math to convert that into a mean radius and CEP. I have met that goal with all of those bullets except Nosler Varmageddon (a frustratingly close but not close enough 0.26 mean radius). But, I am so darn close I'm using up my inventory of those bullets on them anyway.

Coyotes are bigger, so if it works for a gopher, it'll work for them too (although I tend to limit myself to 200 meters on them purely to make sure the bullet has enough "wack", a.k.a. terminal energy, to kill them cleanly.

View attachment 1608218

Thank you for the reply.

I completely agree with your assessment of an acceptable group size. No need to try for competitive short range benchrest groups when your target is a ground squirrel or prairie dog.

I also agree with your method of achieving the group, ie find a load that remains in that group size , even after barrel wear.

I Have been shooting heavier bullets, Berger 64 gr Varmint (no longer made) in my 22-250, and 88 gr Berger Varmints in my 6BR. I have experienced erosion rates like yours, however I do seat the bullets in the lands, typically .007 to .010. This requires me to keep track of the "touch point", which I do using the Wheeler method, which is very accurate.

I will have to try some "jumped" seating trials on my next rebarrel job. The jump advantage being large lots of reloaded bullets may still work. When seated into the lands, accuracy decreases near the end of a 300 to 500 round batch of reloads.

 

[searcher]

I wish I had kept track of (measured) throat erosion on some of my varmint rifle barrels, more so for interest than needing to know. I find it very interesting as to how often some folks replace barrels such as .222's and .223's. We know that throat erosion is influenced by several factors, including the burn rate of powder, type and weight of bullet, etc.. I do know that when I use slower powders (like 8208XBR) the brass comes out hot enough to burn holes through my brass catcher, whereas the brass is pretty cool when I use W748 and LT32. When I borescope, I do see the progression of the throat erosion, but I see far more drastic changes in the barrel wear the first 4" to 10", depending on how many rounds the tube has. I have a number of .223 and .20 Practical barrels with over 7,500 rounds on them on it and it defies common wisdom as to how they "should" shoot. Lots of erosion, lands worn down about 10" very noticeably - though not much cracking. Most have had a steady diet of LT-32, W748, 8208XBR or Reloader 7, depending on the tube. I've not messed with the loads after initially getting them dialed in for most tubes. I find this interesting as the obvious change in distance to the lands and the reductions (avg. 80- 120 FPS) of velocity haven't changed the accuracy of the loads much at all - maybe 1/16th to1/8" MOA. After I run through the remaining loaded ammo on my highest-count barrel (20P), I'm going to up the charge to get back to the original velocity - just to see if there is any improvement. I find it tough to retire great shooting barrels that are still up to the task of whacking those squirrels. When one shoots maybe 500 ++ rounds at a time on a squirrel shoot, loading to the lands presents problems as you can lose a major tune in the middle of the shoot. Given most pre-load for such shoots - that is one good reason to jump, which most light varmint bullets are happy with anyway. I'm guessing those that change barrels most often are those loading heavier bullets to the lands and feel they have encountered too much erosion? I know a lot of guys settle on a good load with a powder that didn't really shoot so good through the entire powder ladder. Just at certain "sweet spots - or nodes". That sweet spot eventually ends if the powder wasn't forgiving through a broad spectrum of powder charges.

 

[Murray Brook]

The only erosion #'s I have are for my 6BR Norma. When the rifle was new using the Wheeler method of finding touch my CBTO with a 64 gr Berger Column was 1.727. After a little over 500 rounds the CBTO at touch was 1.738 . At 1000 rounds the CBTO was 1.758 and now at 2000 rounds the CBTO is 1.769. It seems the erosion hasn't kept a steady pace. Keep in mind that I am not a machinist but measure the best I can.

 

[Webster]

Wow.
My 223 barrels regularly are replaced in the mid to high 3K round range. With a bore scope, I can't tell there's been any reduction in the lands. Of course, distance to the lands [mesured with the same 'reference' bullet over the past 10 yearserodes around 2-3/thousandths per 100 shots.

Erosion rounds the sharp machined beginning of the rifling and becomes somewhat rounded giving inconsistant touch numbers. I only readjust touch jump if accuracy falls off. Just try a couple new lengths. An accurate 6BR varmint rifle.