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Old December 1, 2013, 03:31 PM   #1
1stmar
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Throat erosion and barrel wear ??

Can you measure throat erosion over time using an oval comparator and if so is throat erosion directly correlated to barrel wear?
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Old December 1, 2013, 05:30 PM   #2
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Commercial gauges are made for the M1 and variants, but haven't seen them for other chamberings.

Throat erosion is barrel wear- or are you referring to actual wear of the lands?
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Old December 1, 2013, 05:48 PM   #3
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Throat erosion and barrel wear ??

Referring to wear of the lands. I have comparators already just wondering if they can be used to gauge wear and if throat erosion is indicative of lands wear.
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Old December 1, 2013, 07:34 PM   #4
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tagged , I want to follow this
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Old December 1, 2013, 09:07 PM   #5
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Issue for full auto?

The only instances where I have ever read or heard of throat erosion issues involved full auto firing weapons, and Cordite, which was reformulated more than once. I am guessing this is not a huge issue for sporting arms.
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Old December 2, 2013, 04:38 AM   #6
1stmar
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Throat erosion and barrel wear ??

Not sure if that's true. I thought I read where high velocity cartridges ie 22-250 were prone to this.
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Old December 2, 2013, 10:00 AM   #7
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“Can you measure throat erosion over time using an oval comparator and if so is throat erosion directly correlated to barrel wear?”

1stmar, if the ‘you’ in “can you measure throat erosion over time etc..” refers to me, the answer is yes, I do not covet tools, I make tools, I use the tools I make.

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Old December 2, 2013, 10:09 AM   #8
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Referring to wear of the lands. I have comparators already just wondering if they can be used to gauge wear and if throat erosion is indicative of lands wear.
Yes you can, that's exactly what the comparator is doing when used in the same gun over time.

Some cartridges, you'll hardly notice any difference over a few thousand rounds. Others, it'll be detectable over 100 rounds and dramatic under 1000.
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Old December 2, 2013, 10:14 AM   #9
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The long and short answer is yes you can. I use the Hornady LNL OAL gauge for setting the OAL on new bullets for my rifles. If you keep the bullet that you used to originally measure your Max OAL and use it periodically to measure your MAX OAL you will see if your throat of your rifle is getting larger or not and by how much. This can also be done with a cleaning rod to measure the barrel length to see if your rifles throat is expanding.

My concern has always been for my 243 since it is notorious for burning up barrels.

Good luck and safe shooting.
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Old December 2, 2013, 12:03 PM   #10
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As the others said, you can track throat erosion with any overall cartridge length gaging method as long as you use the same bullet. The military gauges allow for throats to wear 0.010" deeper before rejecting them. For match barrels they use half that number.

Does the throat indicate wear in other parts of the barrel? No. It wears much faster. Temperature and pressure are highest when the bullet base is still very near the throat. The rest of the barrel gets the bullet at lower pressure and temperature and with the rifling already swaged into it by the throat, so there isn't nearly as much friction being applied there. It is a common practice, when a throat becomes badly worn, to remove the barrel, and use a lathe to set the back end of the barrel profile forward by one turn of the barrel threads, then cut the now-shortened chamber back to length with a finishing reamer to create a fresh throat and chamber surface. The barrel can then go on to shoot well again until the new throat wears out.

Muzzles can also wear because propellant gas carries enough inertia expelling from the barrel to pull a partial vacuum in the bore behind it, which then draws cooled, gritty powder particles and combustion products and dust back in at the muzzle. The bigger the case and chamber capacity and the smaller the bore, the more of a problem this is. It can make it necessary to cut the muzzle back an inch and re-crown the barrel at the same time as the new chamber is cut forward in order to recover the original barrel accuracy. Though, on many standard chamberings that are not overbore chamberings, it may not be necessary to do or the cut back needed may be less than an inch.

I mentioned the above just because the muzzle area is part of the barrel wear picture. Wear in the rifling between the throat and the muzzle is where it is least.
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Old December 2, 2013, 04:03 PM   #11
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Throat erosion and barrel wear ??

Quote:
Originally Posted by Unclenick View Post
As the others said, you can track throat erosion with any overall cartridge length gaging method as long as you use the same bullet. The military gauges allow for throats to wear 0.010" deeper before rejecting them. For match barrels they use half that number.

Does the throat indicate wear in other parts of the barrel? No. It wears much faster. Temperature and pressure are highest when the bullet base is still very near the throat. The rest of the barrel gets the bullet at lower pressure and temperature and with the threads already swaged into it by the throat, so there isn't nearly as much friction being applied there. It is a common practice, when a throat becomes badly worn, to remove the barrel, and use a lathe to set the back end of the barrel profile forward by one turn of the barrel threads, then cut the now-shortened chamber back to length with a finishing reamer to create a fresh throat and chamber surface. The barrel can then go on to shoot well again until the new throat wears out.

Muzzles can also wear because propellant gas carries enough inertia expelling from the barrel to pull a partial vacuum in the bore behind it, which then draws cooled, gritty powder particles and combustion products and dust back in at the muzzle. The bigger the case and chamber capacity and the smaller the bore, the more of a problem this is. It can make it necessary to cut the muzzle back an inch and re-crown the barrel at the same time as the new chamber is cut forward in order to recover the original barrel accuracy. Though, on many standard chamberings that are not overbore chamberings, it may not be necessary to do or the cut back needed may be less than an inch.

I mentioned the above just because the muzzle area is part of the barrel wear picture. Wear in the rifling between the throat and the muzzle is where it is least.
Thanks for the responses guys this is helpful and inline with what I expected. Uncle nick ,Indirectly sounds like there is some correlation i can draw on with barrel wear. If my throat hasn't changed or is within .005-.01 my barrel wear is probably not a concern. Hopefully I have my notes on my 22-250 from when i first started loading.
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Old December 2, 2013, 04:16 PM   #12
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If my throat hasn't changed or is within .005-.01 my barrel wear is probably not a concern.
I wanted to follow this thread just to make sure my understanding of the issue is correct . It is and I would say your not going to need to worry about barrel wear until you do below for the second time
Quote:
use a lathe to set the back end of the barrel profile forward by one turn of the barrel threads, then cut the now-shortened chamber back to length with a finishing reamer to create a fresh throat and chamber surface. The barrel can then go on to shoot well again until the new throat wears out.
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Old December 2, 2013, 06:10 PM   #13
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Throat erosion's a function of gas volume- and bore diameter.
As in, a 30-06 (.30 cal bore) necked down to a 25-06 (.25 cal bore) means you're pushing a similar volume of gas through a much smaller opening- resulting in higher gas velocity, temperatures and erosion of the metal.

As mentioned, the throats will always burn up faster than the lands will wear in this case, just as they'll wear faster from "hot" handloads.
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Old December 2, 2013, 07:32 PM   #14
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Throat erosion and barrel wear ??

Thanks guys probably just saved me 600$ on a new tube.
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Old December 3, 2013, 03:10 PM   #15
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1stmar, In the beginning it would have been possible, the length of the chamber could have been a part of the rifles permanent record. It would have been so easy, the length of the chamber (head space) could have been checked daily, in thousandths on the 03 Springfield.

I have shot out barrels, I have barrels that are worn out. I have barrels that should have been cleaned, those barrels are worn out and eat-up. I do not have worn out barrels that had an exemption, meaning the ware is not confined to the throat, those with shot out throats also have muzzle erosion. Back in the old days shooters have been accused of cleaning their barrels and and removing metal from the muzzle at the same time. I remember grinding the 4 piece rods smooth at the joints and or taping them.

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Old December 3, 2013, 04:13 PM   #16
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Not sure if that's true. I thought I read where high velocity cartridges ie 22-250 were prone to this.

7mmRUM is notorious for this!I have seen throat erosion in as little as 600 shots due to excessive freebore in the 7RUM.
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Old December 3, 2013, 04:28 PM   #17
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I can attest to erosion in my .308 Savage.

I measured the chamber when it had less than 500 rounds through it.
I knew it was a deep chamber but I was surprised by how deep it was.
That complicated my search for accurate loads, because I could adjust the OAL out from 2.800 SAAMI recommendations by 0.080 inches and still not be within 0.020 inches of the rifling.

I found good nodes with certain loads at about 0.030 increments.
I also found that adjusting the OAL by 0.005 caused the node to shift and I had to increase the velocity by 12 to 13 fps on average to get equal accuracy.

Needless to say I spent a lot of rounds learning that rifle and its quirks.
After more than 6000 rounds, it started to lose a bit of accuracy so I sent it back to Savage for an upgraded target barrel. By that time the chamber had eroded quite a bit and I could shoot 168 SMKs with OAL of 2.960 and not be touching the rifling. The chamber had eroded by almost another 0.08 inches before I decided to change the barrel.

The rifle with a new barrel was back from Savage in one month. Thankfully, the new barrel has a chamber that is 0.020 inches short of SAAMI. It should last a lot longer and I won't be able to adjust the OAL so much that I need to shoot loads of ammo to find the nodes.

The first groups I shot with it with SMK and Nosler bullets with 168 and 175 grains averaged 0.501 for all the groups recorded. Not a bad start during break in.

I have another Savage .308 that is relatively new with only 600 rounds through it. The chamber has stretched 0.013 from new.
My buddy has an identical Savage whose chamber has grown 0.030 in 1500 rounds. The erosion per 500 rounds doesn't seem to be linear but you can sure measure it.
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Old December 3, 2013, 04:28 PM   #18
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It's not velocity in itself that does it. It's the two things that tend to go along with hyper velocity, high pressure and "over bore". Most cartridges that are known for being barrel burners have both.
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Old December 3, 2013, 05:42 PM   #19
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Throat erosion comes about in this way. When the powder charge ignites, gas is generated and pressure begins to build. At any given time, the pressure is the same at all points inside the case. One of the first results is that the gas pushes around the bullet and into the case neck, expanding it and freeing the bullet. But the bullet does not immediately begin to move; its own inertia keeps it motionless for just an instant. In that instant, high pressure gas (which includes burning powder) rushes around the bullet and into the barrel throat just ahead of the bullet. There is no way to prevent this, although some powders burn hotter and are more erosive than others, and some bullet shapes can cause erosion faster. Rapid firing, as in an auto weapon, can increase throat erosion substantially.

That gas, in moving past the bullet, increases speed so it acts like an acetylene torch turned into the barrel, cutting away the steel and enlarging the throat. But the gas is also pressing against the base of the bullet, so, eventually, the bullet decides to get off its fanny and start to move. It moves forward into the barrel and seals the barrel; erosion stops.

But as firing continues, and more and more of the throat is eroded away, the bullet begins to skew before it encounters good rifling. The result is a bullet with an irregular shape, and uneven rifling marks. If the bullet skews enough, the base might even be distorted. All of that, of course, causes the bullet release at the muzzle and its subsequent flight to be erratic and the result is an inaccurate rifle, even though most of the rifling may still be good.

But the erosion will continue to move forward, since the fast moving gas will still rush past the bullet even as it moves into the barrel. Eventually, the erosion will be far enough forward that the gas will cool somewhat and erosion will stop. But then, it will be too late to save that barrel.

So, yes, you can have throat erosion without having a worn out barrel so far as rifling goes. But the simple fact is that there is no way to prevent some throat erosion.

As to muzzle erosion, there really is no such thing. Erosion is the product of hot gas, and the gas has cooled enough that erosion stops long before the muzzle is reached. What is called "muzzle erosion" is really either rust due to water entering the barrel, or wear caused by improper cleaning practices.

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Old December 3, 2013, 06:18 PM   #20
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Does the throat indicate wear in other parts of the barrel? No. It wears much faster. Temperature and pressure are highest when the bullet base is still very near the throat. The rest of the barrel gets the bullet at lower pressure and temperature and with the rifling already swaged into it by the throat, so there isn't nearly as much friction being applied there. It is a common practice, when a throat becomes badly worn, to remove the barrel, and use a lathe to set the back end of the barrel profile forward by one turn of the barrel threads, then cut the now-shortened chamber back to length with a finishing reamer to create a fresh throat and chamber surface. The barrel can then go on to shoot well again until the new throat wears out.
I would agree the throat goes first. But I have measured the muzzle erosion on my 308 target rifles, and guess what, the muzzle also erodes. You can measure it, so it is real.

If you do plan to reuse the barrel, remember to cut off about 1/2" maybe more of the muzzle.

And another thing, 308 barrels that gage over "3" can still shoot outstanding scores at 300 yards. Been there and done that. Take the same barrel out to 600 yards, and you can barely hold the ten ring. The further you go out, the more barrel wear shows up. The more barrel wear, the worse it gets. You never come out ahead. I assume it is due to bullet wobble. I respect the quote from David Tubbs who said a "barrel is either shooting better, or it is shooting worse".

Bart B has good data for the accuracy life of barrels based on his vast experience. His criteria is probably based on that 600 yard lifetime, which is a more severe test than the short range.

I met a guy at a long range match, he had a 6.5 - 284 and after 900 rounds or so, it would not hold a 5" group at 100 yards. There are some cartridges that just eat barrels before lunch.
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Old December 3, 2013, 07:24 PM   #21
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Throat erosion and barrel wear ??

The good news I have some decent notes from when I initially got the rifle, including the bullet I did my comparison with. I'll check it out and see. The bad news is I bought the rifle used so I don't have an accurate picture of the life. It still shoots well, most groups are between .4 and .7". Average is probably about .55 for 5 shot groups. It shows some real promise but can't seem to keep it at .3 and below. I did just rebarrel a 30-06 and I recently bought an ar and an m1a so I can keep good records on those. I may end up rebarrelling anyway. I love the cartridge and think I can get more out of it.
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Old December 4, 2013, 07:36 AM   #22
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I had no ideal the vacuum in the chamber and barrel had enough pucker power to pull moisture back into the barrel and chamber fast enough to ware the the muzzle, after taking a few moments to think about it, I wonder?

I wonder if the person that made that story up thought about heat and travel, the heat in the chamber and barrel would prevent pucker power (vacuum), for the atmosphere in the chamber and barrel to equalize the air returning would have to be heated or the chamber would have to cool first. Then there is the value of skid, the bullet starts our at ‘zero’ feet per second and exit the barrel in a blur.

The bullet, when leaving the barrel, is getting out of the way as fast as possible, as soon as the barrel leaves the barrel there is a floral looking arrangement of hot high pressure metal cutting gas escaping behind it, I know, time as a factor is difficult to grasp, but the high pressure behind the bullet wants out. as soon as the bullet clears the barrel the gas flares into the floral arrangement.

Many years ago a photographer won an award, he took a still of a projectile leaving the barrel, complete with moisture visible from compression in the form of a smoke ring, gas escaping that was so defined the rifling could be counted, anyhow once the ordinance department saw the picture they had to reinvent everything they thought they knew about the event.

The photograph was art, that is the reason the picture has not been seen in the world of guns. Legend has it the ordinance department took a year to sort out the events of the projectile leaving the barrel, I have this book of photographs that have won awards, one of the pictures is most impressive, nothing about the picture would be visible if time was not a factor. Even then the picture would not have been possible if the picture had not been taken of a humid day.

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Old December 4, 2013, 07:44 AM   #23
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Bullet sealing the bore and gas does not pass the bullet. In the picture the first event was gas escaping the bore ahead of the bullet, and I had to ask: “How did the gas get out of the barrel ahead of the bullet, if the gas did not pass the bullet”?.

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Old December 5, 2013, 11:31 AM   #24
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Gas molecules can pick up inertia, same as any other moving molecule, and they will flow forward until opposing air pressure overcomes that inertia. When the gun bore and atmosphere are in equilibrium, the pressure in the bore is the same as outside, and molecules don't move in or out of the bore because the equal pressures neutralize one another. But when you add forward inertia to the molecules in the bore, it takes time for the outside pressure to decelerate it to overcome that inertia, during which molecules continue to flow out of the muzzle, causing a partial vacuum. The actual form will be damped ringing as the inward rush to fill the barrel picks up its own rearward inertia that interior pressure then has to re-expel.

I once read a study of people resting and exercising at elevated atmospheric pressure (almost 7 atmospheres). The bags the subjects exhaled into tracked the volume of inhaled air when they were at rest, but would momentarily expand up to 20% over that volume when the subjects were exercising and breathing hard. In trying to figure out where the ghost air extra air was coming from, it was finally worked out that this stretch was due to air inertia acting on the bottom of the bag. And this was just air at both pressure and velocity much lower than you see in propellant gases in a gun.
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Old December 6, 2013, 09:26 AM   #25
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Reminds me, I was using an electrical appliance and decided instead of flipping the switch I would simply pull the plug, and then? The thought occurred, the electricity is coming from the power plant at a speed just slower than the speed of light. Then the conundrum, pulling the plug can not stop the flow of electricity if it is traveling that fast. The electricity headed to my plug had no way of knowing I unplugged the appliance.

In the picture there was a concentric floral behind the bullet, the floral was at a 90 degree angle to the bore, and I wondered, what effect the hot high

pressure metal cutting gas had on the radius of the bore at the muzzle, the change of direction had to have an effect on the radius when it made the turn.

What made the 90 degree turn? Matter, matter has weight and takes up space matter can be abrasive, again, I do not have a barrel that has a shot out throat that does not have all the rest of the problems like worn rifling and taper at the muzzle. I have seen the test results of the first atomic bomb, initially the blast lays everything out and down, then suddenly and without warning it appears everything changes direction and everything appears to get return when the vacuum/low pressure is filled.

7 atmospheres of air is equal to 105 psi = water at about 242 ft, meaning that is more than enough pressure to blow the chest cavity like an old tire, unless the pressure was equal on both sides.



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