Overbore Index Chart & 26 Nosler

[Engineering101]

Have you guys seen the chart that estimates how overbore a cartridge is over on accurate shooter.com?

"Overbore" Cartridges Defined by Formula within AccurateShooter.com

I have a 26 Nosler down at the gunsmith getting put together right now and with all the overbore talk I thought I would try to quantify how overbore it is. Some comparisons NOT on the chart:

338 RUM = 1,255
7mm WSM = 1,298
300 RUM = 1,578
7mm RUM = 1,789
26 Nosler = 1,852

Looks like the Nosler is the winner but only by about 3.5% over the 7mm RUM and 17% over the 300 RUM so it is not like it is completely insane. So there you go, the 26 Nosler should be about like the 7mm RUM for barrel life. What kind of barrel life are people getting from their 7mm RUMs?

 

[Mikecr]

You might as well forget barrel life.

You should have barrels finished several at a time,,

 

[HARPERC]

You might as well forget barrel life.

You should have barrels finished several at a time,,

At least one to shoot, a spare, and one in the shop!

It's all good, my only objection to "barrel burners" is how behind the good smith's get. If I could do it myself I'd burn one just because.

 

[Engineering101]

harperc

I should have used a Savage action - only takes 10 minutes to put on a new barrel.

By the way, I'm a graduate of University High. Are you a Spokane native?

 

[jrsolocam]

What's funny is that Weatherby, Dakota & Lazzeroni shorts are now docile conservative target rounds, lol.

 

[barnesuser28]

Probably wouldnt be a bad idea to melonite your barrel, my guess is around 600 rounds of accurate (1/2 moa) barrel life if you take good care of it.

 

[HARPERC]

By the way, I'm a graduate of University High. Are you a Spokane native?

California transplant in 1978.

 

[J E Custom]

Have you guys seen the chart that estimates how overbore a cartridge is over on accurate shooter.com?

"Overbore" Cartridges Defined by Formula within AccurateShooter.com

I have a 26 Nosler down at the gunsmith getting put together right now and with all the overbore talk I thought I would try to quantify how overbore it is. Some comparisons NOT on the chart:

338 RUM = 1,255
7mm WSM = 1,298
300 RUM = 1,578
7mm RUM = 1,789
26 Nosler = 1,852

Looks like the Nosler is the winner but only by about 3.5% over the 7mm RUM and 17% over the 300 RUM so it is not like it is completely insane. So there you go, the 26 Nosler should be about like the 7mm RUM for barrel life. What kind of barrel life are people getting from their 7mm RUMs?

I am by no means a expert on this subject and have read every article I could to understand the relationship between a case/cartridge and barrel life. PO Ackley was one of the first to address this
and do extensive testing to understand this issue.

His writings are very well known and tested/proven so he is one resource that I truly believe. With that said, Hear is the way I understand all of this.

First he states that there are no ONE formula for deciding barrel life based on case capacity compared to bullet diameter.

There are to many variables effecting barrel life.

There are at least 6 different factors that effect barrel life and determine what is considered
"Overbore"

Type of powder, bore grove diameter, twist rate (Pitch) sectional density of bullet, allowable
pressure and case shape.

The shape of the case will have a large effect on erosion even though they have the same volume.
In his writings he states that he found that the longer cases with lots of body taper caused the powder to have different burn characteristics that the short straight body cases with the same volume forcing the powder to compress and be pushed through the neck causing more throat erosion from the un-burnt powder. The short straight sided cases burned more powder before it left the case with the same amount of powder because of the shorter column of powder and the greater shoulder angle. (30 to 40%) that don't compress the powder and force as much out of the barrel before it is burned.

He also addressed the effects of twist rates on barrel erosion. Using the same cartridge, volume,
bullet weight in barrels of the same length, but with different twist he discovered that 30 to 35 %
of the energy needed to push the bullet down the barrel was used to rotate the bullet depending
on the twist rate. To prove this they tested a barrel with No twist (Smooth Bore) trying to get 100% on the energy to propel the bullet. The smooth bore did get more velocity but lost accuracy. So a fast twist barrel will need more powder/pressure to get the same velocity as a slow pitch barrel of the same case capacity and powder charge.

What all of this tells me is that some cartridges may not be over bored but can still decrease barrel
life, and some of the large "Over bored" cartridges may not be as bad as though because of there case design, the type of powder used, the type of bullet, the twist rate ETC.

With all of this, I don't worry about barrel life and just plan on a barrel change when and if it needs one.

There is a lot more information available and one should read as much as possible to understand
the effects of all factors and decide which way to go.

Conclusion = The 26 Nosler may very well be a reasonable barrel burner and based on the information available there are worse barrel burner cartridges in use.

Don't let it get Hot, keep it clean and don't over load it and you should get reasonable or good barrel life with it.

J E CUSTOM

 

[Engineering101]

J E Custom

That is some really good info on barrels. Thank you for taking the time to explain. It did however make my head hurt just thinking about all that. It is pretty clear that how you use a rifle can make it worse. My nephew was working a p-dog field over in Montana on a warm day. They were out there thick and he kind of lost track of his rifle - 220 Swift. Later he checked the distance to the lands in case there had been some wear and the bullet went 6 inches up the barrel. He had literally melted the first 6 inches of the rifling and needless to say he found the accuracy was gone too. He has 4 young kids and a rebarrel is not on the top of the priority list so the rifle is basically useless. I'm going to do as you suggested and shoot the 26 Nosler until it needs a rebarrel but since it is a hunting rifle that might take a while. I've got a Savage LRP in 260 Rem to use if I just want to throw shot after shot which it can do without undue harm.

 

[snowpro440]

Have you guys seen the chart that estimates how overbore a cartridge is over on accurate shooter.com?

"Overbore" Cartridges Defined by Formula within AccurateShooter.com

I have a 26 Nosler down at the gunsmith getting put together right now and with all the overbore talk I thought I would try to quantify how overbore it is. Some comparisons NOT on the chart:

338 RUM = 1,255
7mm WSM = 1,298
300 RUM = 1,578
7mm RUM = 1,789
26 Nosler = 1,852

Looks like the Nosler is the winner but only by about 3.5% over the 7mm RUM and 17% over the 300 RUM so it is not like it is completely insane. So there you go, the 26 Nosler should be about like the 7mm RUM for barrel life. What kind of barrel life are people getting from their 7mm RUMs?

Just curious as to where the 28 and 30 Nosler will fit on your chart ?

 

[Mikecr]

Anyone p-dog shooting-at (it ain't hunting) with a 220swift, should have known that would be the end for it's barrel.

 

[Trickymissfit]

I read Ackley's work several times, plus two or three others. All were similar, and yet a little different. What I like about Ackley is that he spells out what the bore can handle (powder burn), and then you can take it from there. What Ackley didn't really get involved with was things like accuracy (as in target shooting), and to a certain extent cartridge case shape. He does mention some short case designs that are very similar to the WSM series, but doesn't go much further.

Looking at the chart, I see some interesting things, and others that make little sense. A fine example is the 6PPC. Most folks think it's a near perfect case capacity for the 6mm bore. Although others have actually shortened the case up to .100" with great results. If one looks at the chart he might be turned away, when it's a well known shooter.

Ackley may have been the first to spell doom on over bore rifles. He pointed out the loss of accuracy, barrel life issues, and one thing seldom discussed; over bore rounds are much harder to develop hand loads for as they are somewhat finicky. He also points out the issue with high volume to bore ratios, and being down loaded. Few others will even speak of this.
gary

 

[Greyfox]

Being an avid 6.5-284 shooter, the single most frequent comment tht I will get is "Oh, that's a barrel burner!". The Accurate Shooter chart really gives some perspective on the rational for this reputation. This reputation that originated with the competitive target shooting crowd was established because their point of comparisison were the calibers that were in the mid to lower hundreds in the white portion of the chart. Because of this the barrel burner reputation was established. In reality, and what has been my practical experience is that the 6.5-284's falls somewhere in the middle of the high performance hunting cartridges. As a comparison, my competitive 308 barrels that were shot hot, in high volumes would begin to loose accuracy(..25-.5MOA to .5-.75MOA) at around 2300 shots. I expect the 308's would have held out for +3000 shots if shot cooler. My 270WSM, 300WBY, when shot cool, would start to see accuracy erosion at 600 rounds. I would expect the 260 Nosker to fall in this range. 6.5-284's which are in the mid range of the orange section of the chart, seem to display accuracy erosion, when not allowed to overheat, in 1100-1300 range. There are some very popualar hunting cartridges that will burn a barrel faster than the 6.5x284. This was not a scientific study, just observations, but seem to be consistent with a few of my buddies experiences. While I will consider barrel life with my target rifles, I never paid too much mind to it for hunting,
particularly since I started shooting at longer ranges.IMO

 

[dkhunt14]

There are other factors beside the case volume although volume to bore doesn't help. Type of powder and rate of fire is important. One of the things that is overlooked is the Turbulence Point. Which has to do with neck length versus shoulder angle. It is one reason most don't last as long. Matt