Home Forums Hunting & Shooting Hunting Rifles Barrel heat and throat erosion

Ever since Charlie brought up this article for Precision Shooting about barrel heat and throat erosion, I have been chomping at the bit to get it. The mail being what it is, I didn't get the April PS until today. I eagerly read the article as soon as I could.
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<br>To summarize, Mr. Murphy said that shooting the barrel when hot won't cause throat erosion. He said large doses of slow burning powder, friction, the type of barrel steel, shallow throat angles, and the number of shots fired are the factors. This does make sense. I am trying to reconcile it with my experience.
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<br>I had a stainless Sendero in 7mm Rem mag. I would shoot a string of five, then let the barrel cool. In 400 rounds I eroded the throat .040," the accuracy went to pot along with the velocity. The throat should not have eroded that fast. The barrel was 416 stainless, which acceding to the article, is supposed to be the most prone to throat erosion--it may have also been a bad (ie, high sulfur content) batch of steel. The 7mm Rem mag has a throat that seems to promote erosion. That may be due to a shallow angle. Another factor is the turbulence point of this chambering is outside the case neck. This means the hot gases and powder particles are directed to the throat of the rifle instead of the neck of the case. Most of the rounds I fired were loaded with large doses of slow burning powder and heavy bullets. I guess there is enough here to say that shooting it when the barrel was hot didn't hurt?
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<br>I also had an M-1 Garand in 30-06 with Barnett (Douglass blank) chrome moly barrel. It eroded the throat .020 in about 300 rounds. I would fire this gun 10 times in a row before letting it cool. The 30-06 case is not over bore, the barrel was not stainless, the turbulence point of a 30-06 is inside the case neck, and I used medium weight bullets with a medium burn rate powder. I do not know what the throat angle was. I've read where the first few rounds fired through a new barrel "nitride" the throat. Before this occurs, the throat can be easily eroded, but after, the erosion slows dramatically. I didn't do enough testing to verify that with my 30-06.
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<br>I also has a stainless Sendero in 300 Win Mag. The throat eroded .010" in 825 rounds. I seldom shot this when it was hot (have done it once or twice).
<br> I used slow powders with medium weight bullets. The 300 Win is not as overbore as a 7mm Rem mag. I don't know what the throat angle is, but the turbulence point is outside the case neck.
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<br>I also have some custom barreled guns--all stainless. One barrel was a Shilen, one an Apex, and all the rest Pac-Nors. I haven't had any throat erosion in these guns yet. I seldom shoot them when they are hot (have done it once or twice). The 30-06, 340 Wby, 375 Ack, 416 Rem, 338 Win, were/are not overbore at all. The 338 Ultra is about like a 7mm Rem Mag in terms of being overbore. I also have another 300. I had/have less than 300 rounds through any of these rifles. Maybe the quality of stainless steel is better than the factory offerings, and I know John Rick's work is better than factory work. All but the 300 have the turbulence point inside the case neck. Perhaps the throats all have sharper angles.
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<br>Maybe I can shoot them hot. I wonder which gun I should try it on?
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<br>Blaine

The Black non politically correct one
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<br>Mike

Blaine,
<br> Good to see you back. I've come to miss your novels.
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<br>Regards, Matt.

Mike,
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<br>The black, non-politically correct one will certainly get that treatment, but I wonder if the chrome lined barrel will make a difference?
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<br>Matt,
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<br>I miss posting and lurking. I have about one more month to go here in Saudi, then I'll be able to post at a more "normal" level.
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<br>Blaine

Hi Blaine
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<br>How is the turbulence point of a cartridge determined ?
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<br> Knowing this could be a real asset in choosing a new rifle or chambering
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<br>If you try it with the 375, then I'll know how much to baby mine

Canon,
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<br>There is some math involved and I don't have to fromula with me over here in Saudi. You could get an idea by looking at an accurate cartridge drawing, and see if a line extending along the shoulder intersects the case neck or xits the case mouth. I picked the 6mm Rem over the 243 Win for this very reason.--the 6mm typically has a longer barrel life, even though it holds a little more powder.
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<br>BTW, your 375 should be okay. It isn't over bore at all.
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<br>Blaine

Blaine
<br>Looking at drawings in a loading manual, the throat angle is obviously in favor of the 6mm over the 243. When comparing the 25-06
<br>and the 257 Wby. the 257 would appear to be the one with more longevity, so I assume this is where the formula comes into play.Would like to learn more of this concept, so maybe when your home you could share some more info.
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<br>I'm sure you will enjoy the 6mm. The throat on mine just gave out after sixteen years, but I'm going to try a 243 AI for the fun of it. Will this improve the barrel life by changing the shoulder angle ?
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<br>Just kidding about the 375 . As much as I like this round, I doubt I'll shoot it enough to wash this one.

Canon,
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<br>The Wby chamberings are interesting. The double radius shoulder would seem to keep powder from sandblasting the throat. All except a few custom rifles in Wby chamberings are freebored. I don't know if the freebore helps in this regard. The only way to tell would be to measure the throat, and to do that you'd have to have very minimal freebor--if any at all. Even so, he 257 Wby is a lot more overbore than the 25-06, so it follows the 257s barrel life--as experienced by many--would be shorter.
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<br>Based on turbelence point theory, the 243 AI would indeed have a longer throat life than the 243 Win. I don't know of anyone who has any actual expereince with this though.
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<br>Blaine

Blaine
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<br>I picked a poor example with the 257. According to an arcticle recently, the writers formula for overbore has the 257 at or close to the top, for being overbore.
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<br>Something like the 7mag compared to the 7X61 S&H would be a better choice. The shoulder lines of the 7 Rem mag would indicate it was harder on barrels than the Sharpe and Harte. After comparing these two rounds, it occured to me there could be another reason besides the turbulence point, for the faster throat errosion, on some cartridges. Everyone is theororizing on what takes place in the throat causing premature wear, so I'll throw my version in and see what you think.
<br>No doubt higher pressures create more heat, and when the throat material is hot, it is much easier for the unburned powder to have a sandblasting effect on the rifling.
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<br>In the 7 Rem mag, because of it's funnel shape, it has powder unignited following the bullet down the bore. The powder directly behind the bullet, with it's deterrant coating, is hard and abrasive. If this coating hasn't already burnt off, as it passes the throat, it wears progressively more with each shot. Now every barrel experiences wear, but the Sharpe and Harte cartridge, with is sharper shoulder, has the ability to form a bridge so to speak. This allows the powder to have a more complete ignition, inside the case. If the hard retardant coating has burnt off the powder granules, before the unburnt powder follows the bullet down the bore, it would be less abrasive to the barrel material. Make any sense????
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<br>I'd proof read what I just wrote, but I ran out of smokes an hour ago. I'm tooo dizzy to read it , and don't feel like typing this again so ...........bye

Canon, The Sharpe and Hart also has a longer neck than does the 7mm Rem mag, which when coupled with the sharp shoulder, keeps the powder granules from blasting the leade. I imagine that the shorter fatter case does promote better powder burn and that may be one factor.
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<br>BTW, an east formula to figure the max "efficient" case capacity vs bore size is bore diameter squared x 1000. IE a 308 bore works out to about 94 grains of case capacity. Using this as a reference, you can then divide that number by a case's capacity to get an idea of it's relative efficiency. I built a spreadsheet to do that.