Can someone explain throat erosion factors.
For example,is it more prevelant in a .264Win mag than a .338winMag? considering they burn about the same amount of powder in the same size case? does the smaller restriction of the .264 throat have some contributing effect to greater erosion? ie; smaller dia. (surface area)throat cops more intense heat due to simple physics/thermodynamics? (less area to share the heat distribution, creates accelerated destruction)
So would throat erosion in a .17rem compared to .223rem be like comparing a .264/338?
Is it more the ratio of bore size to case capacity rather than actual case size?
Ie; does .17rem have the same degree of throat erosion as the .264win or .257weath?

Throat erosion is a product of temperature and pressure,not only the peaks of each but the duration as well.In other words given the same peak pressure and temperature the larger capacity cases provide this temperature and duration for a longer time period which causes more throat erosion.Given two cases of equal capacity but different bore size,the smaller bore will suffer more throat erosion.Continually firing a rifle and heating the barrel to high temperatures also increases throat erosion.To extend the life of your barrel,let the barrel cool between groups.

I'm inclined to give credit for the process to pressure, augmented by chemical corrosion. Not a popular opinion but unlike most it has some support.

Some of the characteristics of steel:

Don't higher temperatures and higher pressures go hand in hand in rifle cartridges?

Yes they do, but whether the correlation is linear I don't know. I don't debate that temps. are higher, I just think it is irrelevant for the most part. If you look at the surface of a sectioned barrel, or bore scope of the throat region you typically see an alligator hide texture in the steel, in the form of many many very small cracks. This is NOT caused by temperature.

Wouldn't rapid heating and cooling would cause stress cracks of that nature?

DD, how hot are the gases? I accept that the entire mass of the barrel is not getting near the temperature where significant softening would occur, but the outer few atoms are, and such is the slow process of erosion...a few microns at a time over a long period. Increase the start temperature by shooting a hot barrel, and the effects are deeper for each shot. High pressure gas just isn't gonna hurt barrel steel enough at room temperature. Even accounting for microscopic particles of grit from primers, powder contaminants, etc. just doesn't add up to enough in my admittedly unprofessional analysis.

The texture you see in a bore scope would be as easily explained as the similar effect blowing sand has to a rock. It is just a classic erosion pattern, not particularly indicitave of the method by which the erosion occurs.

From that little bit of twisted logic I am in the camp of both T and P being critical factors inasmuch as they are tied together by the thermodynamics and the fact that there is some abrasive in each shot and the P (velocity really) will have a big effect on that!

As to the original question, it is basically (oversimplified) a ratio of volume of gas passing through to the surface area it abrades...or in rough simple terms as case volume goes up or bore diameter goes down erosion goes up. How fast each affects the erosion is a whole 'nuther lively debate!

The dictionary definition of erosion is 'a wearing away' This is NOT what happens in a barrel and is NOT "erosion".There was a fine article in the American Rifleman years back and I wish I had a copy .It included an excellent metallurgical study of the problem.The barrel steel absorbs carbon fron the gases of the burning powder. Therefore 'carburizing' the steel.The higher carbon steel in heating and cooling then cracks causing the 'alligator' pattern cracks. The way to reduce the problem is to keep the barrel cool by leaving time between shots. So it's not erosion and not abrasion.Higher pressure and higher velocity rounds usually cause more problem.

CDH, carburizing is an issue to some degree as I understand it, but there are more insidious issues at hand in the form of chemical reactions which invade microscopic cracks in the barrel and lead to corrosion.

The issue of heat is secondary IMO, and I'm being generous. I need to dig up some of my personal 'archives' so I can better explain(read: not step on my unit) what is going on there, so I'll post back on the subject a bit later. The point about pressure is this. There is a direct correlation between the location of the cracks as relates to pressure curves in gun barrels. There is also a direct correlation between peak pressures and the magnitude of the cracking, in other words, a .22-250 will have this problem in fairly short order, a .25-20 will not. The location of cracking in the barrels is generally confined to the first few inches of the rifling ahead of and including the throat. This would not be the case if gas velocity or charge particles caused the problem as their velocity is relatively low at that point. On this point I am in complete agreement with Mete, it is NOT erosion or abrasion.

A few final points before retiring to my chambers for specific info on the subject. The duration and magnitude of the temperature curve is too brief to transfer adequate energy to vaporize steel. The barrel EXPANDS(diameter) slightly from the pressure. More presssure=more expansion=more cracks. Again, the cracking commonly found in barrel burners is located in the region of maximum pressure. A final point about temperature. While I realize machine guns are often built with stellite chamber inserts to address this issue(in part) I note that they function through thousands of fired rounds with little ill effect. Surely their barrels get much hotter than sporting arms, no?

I'll be back in a bit on this.

Woodjack: You're already on the right track when thinking Thermodynamics.

DD and Wink: Temperature and pressure are linear when considering a specific powder as the consideration. Some powders will burn hotter than others at a given pressure, but that is powder specific.

DD: that is a very telling graphic you posted and is self evident to what occurs with steel alloys exposed to various temperatures. Erosion is probably a dubious term, but since the throat encounters the highest temperatures in a barrel, I guess somewhere along the line someone coined it before some of the more sophisticated diagnostic techology arrived. Erosion denotes a physical condition that occurs over a sustained period of time, when cartridges like the .264 Win Mag didn't need a lot of time to burn away barrel steel and why Winchester went to stainless to counteract the condition. A similar event occured with the .357 Max in revolvers, but Ruger was slow to react whereas Dan Wesson jumped right on it!

Good Stuff!

A bit more on the process. The bad boys in the chemical process are Hydrogen and Nitrogen, leading to hydrogen embrittlement(more cracking), and various nitride/nitrate(?) compounds resulting from the powder burn.(see graphic above) It is entirely likely that the combination leads to REDUCED bore diameter although I understand that frictional abrasion(bullet) and ablation(proper term for heat effects, if any) may, under some circumstance counter this to a small degree. This phenomenon occurs because once cracked total bulk volume increases and well, it's gotta go somewhere.

The problem with the reduced bore diameter is found in the fact that a typical soft core bullet once obturated, and having passed thru this region of the barrel so affected, will no longer be propelled by sufficient pressure to cause it to obturate again, and accuracy suffers.

This info is distilled(love that word) from some of M.L. McPherson's writings, and when I have time I'll respond with more detail. I have some family backgroud noise in progress, gotta run.

DD, it's been a while since I used my chemistry edumucation, but IIRC most checmical reaction rates are heavily temperature dependant. It also seems that the flame temperature of modern small arms is well in the white heat range of your graph. Even if that heat is of small enough time duration to only penetrate a tiny amount, it will have a significant effect on the surface it contacts...but max pressure corrolates well with max temperature in a barrel during combustion. That makes it hard to tell which is the culprit directly. Also, I seem to remember that carburization only occurs at a fairly high temperature. I am well familiar with hydrogen cracking, being an engineer in the petrochem industry...though we usually get it in the form of hydrogen sulfide. Nasty stuff! Good point...being in contact with a rapidly oxidating gas could easily lead to corrosion type attacks...again the rate of that should corrolate with the temperature though.

As for worrying about MG's, well they aren't known for their stellar accuracy anyways, but what is the life of an M60 barrel at sustained full auto? I bet most target shooters chuck their barrels at a much lower level of accuracy degradation than your average government paid military armorer...and one corporal with a hankering for some rock and roll will put more rounds through a MG in a day than the average shooter in a lifetime...present company excluded of course!

Fun stuff! Now where can I rent a M60 and buy a few belts...all in the name of science of course!

After all the science, will the .17rem have the same level of Erosion, as a 264,257weath.? ie;if the restriction of the neck is in same proportion as the larger case will not heat/pressure and exposure time be similar?

Ahh.. Arrhenius factor (exp(-E/RT)) relating rates of reaction given a 'barrier' energy by kinetics considerations. Ablation is likely a good word for SOME of the process going on, too. The surface molecules sublimate (evaporate) in the high-temp and high-speed gas flow. You're right that small barrels and high temps conspire to let that happen. Some of the other things, like carbonization (adsorption -- binding to the surface -- and alloying -- going into "solution" in the metal) can all be going on at once. The "annealing" transition reflects the general phase transition dependence expected for increasing impurities (increasing tin in lead decreases melting temps, too).

But -- one thing I experienced with a barrel that suffered from it (a .25-06) was that the throat felt "rough" when I put a brush through it -- even when the rest of the barrel felt "clean." The other thing I found was that I was unable to get any accuracy from it. "Overbore" cartridges (hot magnums, even .25-06 qualifies -- fast .22's like the Swift and .22-250 qualify) tend to be more prone than others. Shooting hot will do it much faster. But barrel metalurty is much better than it used to be, and these calibers last much longer than they used to.

Figure if you're getting into it, you've likely spent a lot of $'s on ammo already; send your gun to Hart (other barrel makers will do it too) for rebarreling if you like (I'll plug them: they made me very happy -- which is how my old .25-06 became a .35 whelen). A friend of mine with a problem barrel had his Savage bolt rebarreled by the manufacturer at a wonderful price!!

Hope this helps!!

Dan

quote:

Originally posted by CDH:
...Now where can I rent a M60 and buy a few belts...all in the name of science of course!

Hey CDH, One option would be the Knob Creek Range just Southwest of Louisville out toward Fort Knox. (502)922-4457

email - the creek@sprynet.com
http://www.knobcreekrange.com
http://www.machinegunshoot.com

They have a HUGE Machine Gun Shoot out there each summer. Only had one little girle get killed in the last few years when her Father's Machine Gun got loose and (I think) fell on her. Otherwise, it is interesting to watch all the stuff get blown away down range. They use the 400yd range with some stuff to shoot at all distances.

The range is ALWAYS packed with folks even during mid-week, but most of them aren't shooting Machine Guns. Had a guy shooting a 50BMG a few stations down one day with the "Tank Style" muzzle brake. He didn't seem to be flinching at all, but it sure had me hopping around.

CDH, in answer to your question about temps., Dr. Ken Howell indicates they are in the range of 3000*F in high intensity cartridges, this from "Designing and Forming Custom Cartridges". For this class of cartridges barrel time is less than 1/1000th of a second, so accumulated barrel time for 1000 rounds would logically be less than a second. This is far more than required to seriously damage barrels such as the Swift and I do not see this as adequate to allow sufficient heat transfer to cause the problem. Yes, there is some ablation on a molecular/atomic level, but I do not see how it would become visable after so short a time If heat was the causitive agent. Some reports I've read would indicate throat wear becomes measurable after much shorter volume of shooting and this logically confounds the heat theory even more.

Well, it is an interesting topic, and I've thought for some time that the next great leap in shooting science will be metallurgy that deals with the issue. Can't wait to hear(eh? Whadja say?) the muzzle blast of a 100kpsi shot.

quote:

Well, it is an interesting topic, and I've thought for some time that the next great leap in shooting science will be metallurgy that deals with the issue. Can't wait to hear(eh? Whadja say?) the muzzle blast of a 100kpsi shot. Big Grin

We've been pushing at the limits of 'modern' cartridge science for a long time. I never understood why no one ever made the attempt to design/build a higher pressure round for shoulder fired weapons. 65kpsi is all we have, and the law of diminishing returns has kicked in hard on that little limit!

Oh, I think I have heard a 100kpsi shot...once. The rifle didn't survive. Thanksfully it was some ways down the line!