Ogive shape and measured erosion ?

[atlatl]

I have a 6 mm Remington AI that I have been keeping track of the rate of erosion on, and find it interesting that bullets with different ogives show considerably different changes in jam length over time.

As an example, the jam length for 105 Berger hybrids has lengthened 0.027" from new, while the jam length for 95 grain Nosler BTs has lengthened 0.070 over that same time.

I believe it has to do with the shape of the bullet and how it engages the lead, but I would never have guessed that the difference would be so great from one bullet shape to the next. 70 thou erosion in 5-600 rounds is terrible, while 25 thou would be considered not bad- same barrel same amount of erosion, just tracked with different bullets.

Anyone have similar experience?

Thanks

 

[Nature Boy]

Are you using the same bullet to measure the change?

 

[46and2]

If you change bullet lot number it will be different also.
What powder are you using?
My 6ai lasted 700 rounds with Rl26.

 

[Papa Charlie]

In order to be able to accurately measure throat wear, you have to pick one bullet and use it from start to the end of the barrels life. Even two bullets from the same lot will give you different readings. Chose one bullet, make your measurement and record the reading along with the number of firings on the barrel at that time. Place this with the bullet in a plastic bag, mark the bag with the date, rifle and set it aside. Use the same bullet each time and be sure to record the number of firings. You still start to see a pattern.

If you have not done this, then draw a line in the sand, pick a bullet and that is your starting point. You cannot go back unfortunately.

 

[atlatl]

Thank you for the replies, but perhaps I did not clearly state my question.

Using the same Berger 105 hybrid- the throat erosion measures 0.027 from new.

Using the same Nosler BT - the throat erosion measures 0.070 from new

It surprises me that ogive shape would display such a dramatic difference in measured change in throat erosion.

 

[Ned Ludd]

Compare the CBTO at "touching" with a few different bullets and you may sometimes see the same thing. A few of the bullets with similar ogives might have CBTOs at "touching" that are extremely close, even within a few thousandths. Another bullet with a very different ogive profile may give a markedly different CBTO measurement at "touching". Part of that difference may also be due to exactly where the caliper tool insert used sits down on the bullet ogive, whereas in your land erosion measurements it should be a direct apples-to-apples comparison. Nonetheless, changes in the lead angle of the rifling as it erodes and the specific ogive radius of the bullet used for measurement could easily cause differences in the apparent magnitude of land erosion. The difference you actually obtained with the two bullets seems pretty excessive to me, but it is what it is. With the Nosler BT bullet that suggests .070" land erosion, have you had to change the seating depth accordingly to keep it in tune?

 

[243winxb]

Difference in ogive shape difference.

Measure with a flat base bullet inserted into the case nose first, when barrel is new. Save case & bullets for testing later.

The flat base bullet may give a better idea of the amount of erosion.

 

[Webster]

I have a 6 mm Remington AI that I have been keeping track of the rate of erosion on, and find it interesting that bullets with different ogives show considerably different changes in jam length over time.

As an example, the jam length for 105 Berger hybrids has lengthened 0.027" from new, while the jam length for 95 grain Nosler BTs has lengthened 0.070 over that same time.

I believe it has to do with the shape of the bullet and how it engages the lead, but I would never have guessed that the difference would be so great from one bullet shape to the next. 70 thou erosion in 5-600 rounds is terrible, while 25 thou would be considered not bad- same barrel same amount of erosion, just tracked with different bullets.

Anyone have similar experience?

Thanks

6mm Rem barrel coss section after about 5000 shots. The ridge on the left is where the end of the case neck ends. How do you measure touch when there is zero rifling? 1000 shots the edge of the rifling should be gone. The beginning of rifling doesn't just recede. It's totally detroyed.

Increments on the photo.
2mm = 0.079"
4mm = 0.158"

 

[JEFFPPC]

Question to those tracking erosion. Have you done this due to a lack of accuracy or just due to the fact erosion has taken place? No where on here have I seen any mention of loss of accuracy. I ask this because I average 2000 rds a season from a barrel. I never chase erosion. I will occasionally retune a bit with the tuner if I feel I have lost a bit of percision I usually shoot well at end of year shoots. I realize erosion happens, but at what point does it hurt performance. Over the course of a shooting season I have only had 1 barrel that shot well just go away. I did try to retune it but it was done in 1400 rds. Had chunks of rifling gone. Other than that I have given some 2000+ barrels to a friend who has put them on and won with them. So other than telling me my buddy is smarter than me, I am curious why I have heard no mention of accuracy.

 

[LCazador]

Thank you for the replies, but perhaps I did not clearly state my question.

Using the same Berger 105 hybrid- the throat erosion measures 0.027 from new.

Using the same Nosler BT - the throat erosion measures 0.070 from new

It surprises me that ogive shape would display such a dramatic difference in measured change in throat erosion.

My recent question about changing the default 1.5 degree lead angle relates to this. If you start with a steeper angle I think you will maintain that bullet/rifleing contact longer . The 6mm AI is an overbore cartridge that I've played with in the past. I got only about 1000 rounds out of that barrel before it fell short of my accuracy expectations. I've gotten much better life out of my 6X47L 40 degree. Playing with the 6 Grendal now.

 

[rardoin]

Damn @Webster, now everyone needs a Scanning Electron Microscope to evaluate throats. And I though my Hawkeye was expensive...SHEESH...

 

[CharlieNC]

When the barrel is new the ogive angle, associated with different bullets, engages the lead angle which was cut by the reamer. Fast forward, and there is no reason to believe that the eroded angle would be identical to the new as cut angle. So the engagement from various angle ogives would be altered as well.

 

[LCazador]

Question to those tracking erosion. Have you done this due to a lack of accuracy or just due to the fact erosion has taken place? No where on here have I seen any mention of loss of accuracy. I ask this because I average 2000 rds a season from a barrel. I never chase erosion. I will occasionally retune a bit with the tuner if I feel I have lost a bit of percision I usually shoot well at end of year shoots. I realize erosion happens, but at what point does it hurt performance. Over the course of a shooting season I have only had 1 barrel that shot well just go away. I did try to retune it but it was done in 1400 rds. Had chunks of rifling gone. Other than that I have given some 2000+ barrels to a friend who has put them on and won with them. So other than telling me my buddy is smarter than me, I am curious why I have heard no mention of accuracy.

I chase the erosion and I believe if you don't you will not maintain the level of accuracy you started with. I believe that erosion effects are more subtle at short range than at long range of course. Pindell and Palmisano were the first to change from the 3 degree 222 Remington leade angle to the 1.5. The PPC has dominated since it's beginning so it's chamber design has been duplicated on many other chamber designs. Thus we continue to apply the match leade angle to every match chamber out there including cartridges that dissolve that angle within a couple of hundred rounds!

 

[JEFFPPC]

LC, thanks for the reply. More and more I believe all the little things we do and chase that we can not really prove help, pale in comparison to who reads condtions the best and handle the gun flawlessly. But like everyone else who wants to be competitive I will continue to do most of them.

 

[Webster]

Damn @Webster, now everyone needs a Scanning Electron Microscope to evaluate throats. And I though my Hawkeye was expensive...SHEESH...

No reason to complain about a photograph.

 

[rardoin]

No reason to complain about a photograph.

No complaints whatsoever, I was making a jestful comment in light of the current trend of everyone feeling they need a borescope. I also ran a SEM as well as a TEM in my past life as a research scientist so the pic bought back old memories. In times of boredom we would plasma coat and scan all kinds of interesting objects and make wall art for the lab. If I still had access to one I could really have fun.

 

[Webster]

No complaints whatsoever, I was making a jestful comment in light of the current trend of everyone feeling they need a borescope. I also ran a SEM as well as a TEM in my past life as a research scientist so the pic bought back old memories. In times of boredom we would plasma coat and scan all kinds of interesting objects and make wall art for the lab. If I still had access to one I could really have fun.

Thanks for the nice reply. The point I was trying to make was that the sharp edges on the beginning of the rifling are gone with even a small amount of erosion making it difficult to find touch by engraving on the bullet. Also with advanced erosion the bolt closure resistance method probably won't work well. What's your opinion on the different models of the Teslong (spelling) bore scope. The low end is $49 and it goes up to over $100. I worked for one of the worlds largest chemical companies. I am not a scientist but I performed failure analysis of oil related engine, drive line parts and fuel related problems and many other duties for many years.

 

[rardoin]

I've only used a Lyman. and when it failed, bought a Hawkeye. They Lyman (1st gen) had pretty good images but degraded over about 6 months. The mirror for the 45deg view had a very cheap coating that marred with the most careful cleaning...and I am very careful with optical components. Regardless of my issues with the coating on the Lyman mirror it had a good enough image to assess bore cleaning/throat condition once one was acquainted with exactly what they were looking at. I would expect the same of the Teslong.

 

[Ned Ludd]

Webster - the Teslong is in a whole different class from the Lyman in terms of image quality, it's not even close. I used to use the Lyman, then I bought a Hawkeye about three years ago. Last fall, I bought the least expensive Teslong unit as a backup to my Hawkeye, and to have the capacity to capture bore images electronically. How long the mirror might last is a question I can't answer at this point, but the image quality straight out of the box is excellent. If you are thinking of getting one, there is a thread detailing others' experiences with it:

http://forum.accurateshooter.com/threads/teslong-bore-scope.3987243/


With regard to you image of the rifle throat above, I think that over time the lead angle effectively changes from 1deg30 (or whatever it started out as) to a much shallower one as the lands erode further down the bore. Alternatively, one could think of the corner of lead angle as being worn off. If the rifling is completely gone for some distance into the bore, it would be impossible to measure "touching" as the bullet could move freely into the rifling. If it is a case where the leading edge of the rifling is merely worn down, effectively creating a much shallower lead angle, it should still be possible to get a measurement.

 

[Webster]

Webster - the Teslong is in a whole different class from the Lyman in terms of image quality, it's not even close. I used to use the Lyman, then I bought a Hawkeye about three years ago. Last fall, I bought the least expensive Teslong unit as a backup to my Hawkeye, and to have the capacity to capture bore images electronically. How long the mirror might last is a question I can't answer at this point, but the image quality straight out of the box is excellent. If you are thinking of getting one, there is a thread detailing others' experiences with it:

http://forum.accurateshooter.com/threads/teslong-bore-scope.3987243/


With regard to you image of the rifle throat above, I think that over time the lead angle effectively changes from 1deg30 (or whatever it started out as) to a much shallower one as the lands erode further down the bore. Alternatively, one could think of the corner of lead angle as being worn off. If the rifling is completely gone for some distance into the bore, it would be impossible to measure "touching" as the bullet could move freely into the rifling. If it is a case where the leading edge of the rifling is merely worn down, effectively creating a much shallower lead angle, it should still be possible to get a measurement.

My 6mm Rem had over 2" of rifling missing and it still shot a little under 1". Good enough for GH out to 300 yards.