Three 7 MM Bullets - Page 2 - Graybeard Outdoors
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post #11 of 16 (permalink) Old 03-19-2019, 08:59 PM Thread Starter
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gpa&hisguns - That part about high pressure and lower velocity sounds familiar to me. I had a .17 Remington with a lot of throat erosion. The primers started to crater and the velocity was much less than when the barrel was new with the same loads. Is that what usually happens when a barrel erodes? I have had several barrels go bad from erosion but didn't do a before and after check of velocities except for the .17 Remington.
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post #12 of 16 (permalink) Old 03-20-2019, 06:51 AM
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Hard to argue with hard data, but pressure increasing with throat erosion is counter intuitive. I would equate throat erosion as increased lead which usually means lower pressures. I am interested in the physics that is at play here. Can anyone else shed more light on this phenomenon?



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post #13 of 16 (permalink) Old 03-20-2019, 07:36 AM
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It's all about friction. A new smooth bore has very little and very worn barrel has a lot. As I mentioned I did not take the best care of this gun so some of it is my fault. I remember one virgin prairie dog town I put almost 100 rounds through that gun in a hurry. Yup it got hot....real hot. I cleaned it once a year whether it needed it or not with a good copper remover. So the copper build up with the last chrony test was not an issue. Any time you have a big case with a little hole you are going to have a lot of fire going down the bore. A bore scope will reveal flame etching. This 7WSM had plenty of that going on. The fact that the load that I mentioned is running 63,000+ PSI in a new barrel sure doesn't help. You could feel a cleaning patch catching as you pushed it down the bore. Then looking through the bore after running the patch revealed lots of "fuzzies" hanging onto the sharp edges in the bore.

I have a Ruger 77 tang safety model that I started my coyote hunting career with way back around 1970ish. That rifle is on it's third barrel as of a couple of years ago. A bore scope indicated lots of light cracks on the lands before the last replacement. I'm not a physics major just an old guy that used to shoot his guns a whole lot. It is my belief from what I seen that all of that heat or flame etching changes the metallurgy of the bore. Obviously waiting for cooling between rounds and proper cleaning will improve on barrel life. I am guilty of being way more hunter than shooter and cleaning my guns was not my strong point.

I too would like to hear what some of the long range target guys have to say about barrel wear in regards to rounds through the pipe and cleaning methods etc.....
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post #14 of 16 (permalink) Old 03-20-2019, 07:53 AM
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The friction makes sense. If the bore has gotten rougher, than can definitely cause increased pressures. Maybe it isn't the throat erosion issue but the roughed up bore issue causing the increased pressures.


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post #15 of 16 (permalink) Old 03-20-2019, 08:15 AM
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I don't think copper bullets hurt a barrel over a period of time they just wear it out then accuracy falls off

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post #16 of 16 (permalink) Old 03-24-2019, 12:36 PM Thread Starter
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I don't remember the exact velocity loss with the .17 Remington but am pretty sure it went from about 4,000 fps to around 3,600 fps. So the energy loss would have been from 892 ft-lbs to 723 ft-lbs. The energy the bullet attains should be the energy calculated from the area under the pressure curve minus losses for friction and the energy the gases attain in achieving their barrel exit velocity. Anyway, assuming the same pressure curve, the bullet achieves 169 ft-lbs less energy. It would seem that that loss would be atrubutable to friction from the rough throat. And in addition the pressure curve may have more area under it after the throat got rough. I assume this given that it appears the pressure with the rough thoat because the primers cratered after the throat got rough versus primers which did not crater when the barrel was new. So there was probably much more than another 169 ft-lbs of energy lost to friction given more area under the pressure curve unless somehow the pressure dropped more on down the barrel compared to when the barrel was new. This makes me wonder how long it took my .17 Remington to basically start giving .17 Hornet performance?
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