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Normal Topic Heat vs Pressure vs Velocity - effects on barrel wear part four (Read 112 times)
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Heat vs Pressure vs Velocity - effects on barrel wear part four
Dec 2nd, 2018 at 2:22pm
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To explain my comment in part three

As the loaded round sits in the chamber obeying the rules of Physics, gravity causes the nose to drop as much as tolerance in the bolt head, chamber and Leade will allow. If your load is not at least slightly compressed (> 3%) the powder will not be truly evenly distributed in the case. The final contributors to the problems thrown up here are concentricity of the loaded round and dimensional accuracy/consistency of the projectile.

Ignoring, for now, the final contributors and assuming a non-compressed load, this is the basic description of what happens.

The primer is struck and a commonly non-corrosive, non-Mercury, shock sensitive (commonly diazodinitrophenol based very high explosive these days) compound explodes sending oxidisers and incandescent particles of aluminium, zirconium, magnesium or similar into and through the powder charge.

This ignites the powder. The load is non-compressed so there is an air gap in the case and a pressure differential occurs causing turbulence in the case before the projectile is released. In this situation, the turbulence causes rapid dispersal of unburnt and still burning particles serving to significantly increase the speed of the pressure spike and the total heat achieved. Both are very undesirable outcomes in terms of barrel longevity.

As the powder never lays exactly the same way for each fired round and therefore the pressure and heat profiles vary unpredictably, there is now an inherent or inbuilt, but unknown, variability in internal ballistic performance which ultimately degrades accuracy at the target. Depending on the amount of air gap and the temperature at firing this can be as much as 0.2 - 0.5 mil discrepancy at 100 metres.

As the pressure builds, the case expands to fill the chamber space (Obturation) and the projectile is released. (Please note the projectile is never blown out per se, it is always released in a load safe for use in hand-held firearms.) As the case expands, the mixture of hot gases, burnt, burning and unburnt powder and primer particulates all force their way past the projectile (precession) which is still trying to get moving. Several things are happening together now.
1.      As the projectile is released it begins to accelerate forward and also, due to gravity, begins to fall. The nose falls slightly faster than the base causing a rotation (yaw).
2.      In a non-compressed load, due to basic physics and gravity, there is a slight increase in density at the bottom of the case. Therefore there is slightly (tiny difference) more pressure being exerted on the underside of the projectile during the ignition stage of the process. This “kicks” the rear of the boat-tail up, further exacerbating the rotation of the projectile.
3.      As the projectile rotates more gases etc. escape underneath than above and then follow the curvature of the projectile via the path of least resistance. Centripetal force drives the heaviest particulates outward and some “weld” to wherever they contact the barrel.
4.      More particles bounce off the barrel and then collide with and weld or embed into the projectile surface.
5.      The hottest gases begin to carve microgrooves into the surfaces of the barrel. The worst of this occurs at the junction the case neck and Leade/Throat area of the barrel and along the highest velocity path of the gases around the projectile.
6.      Irregular (from shot to shot) resonance is created in the barrel thus degrading external ballistic predictability and therefore accuracy.

A few microseconds later the dropping nose of the projectile contacts the rifling and begins to engrave and the sealing process is underway. Unfortunately, the high side of the projectile has not yet struck rifling and this takes a few more microseconds. The result is that a small portion of the projectile has significantly longer rifling engraving on one side. It is also had to “bounce” of the rifling to square up in the bore causing further projectile yaw and more uneven gas patterns.

Shortly after this the projectile fully engages the rifling and gas bypass (precession) is all but eliminated. The preceding gases and particles oscillate their way up the barrel and exit the muzzle unevenly creating an uneven and destabilising pressure field for the projectile. This, in turn, further degrades external ballistic predictability and therefore accuracy.

The result of the particulates weld or adhering to surfaces is to set up a destructive albeit small media blasting effect. This is repeated and exacerbated from shot to shot. So the more airgap, rotation, precession and oscillation with each shot, the faster the barrel will lose accuracy.
The next factor is the destabilised burn pattern of the gases following the projectile violently disperse in an irregular pattern as they overtake the projectile when it leaves the muzzle. This tens to set up a new yaw and exacerbate any existing yaw created back at the throat entry phase. Again, this varies from shot to shot and is ballistically unpredictable degrading accuracy still more.

The projectile now has to try and stabilise with uneven rifling engraving around its circumference. Think about driving a 4WD through mud first with smooth, street tread tires and then with heavily lugged mud tires fitted. The street tire slips and slides while the mud tire maintains far more even progress.

The uneven rifling engraving is a bit like the tread on those tires. In one area of the projectile (the shortest piece of engraving) you have the smooth street tread not gripping the air much and on the part (the side which hit the bottom) rifling is engraved deeper and longer and grips the air far more.

As the projectile rotates around its axis at up to (and sometimes faster) 200000 rpm the disparity in traction with the air cause the projectile to rotate about the flight axis slightly so its path is actually an oscillation. Depending on other factors individual to the setup, this oscillation may reduce, increase or remain constant. However, it will vary (sometimes considerably) from shot to shot with more problems for accuracy. Almost inevitably this process set up a degree of yaw as well.

Apart from the obvious accuracy issues which are significantly increased with concentricity or projectile dimensional inconsistency, the issues discussed above are always going to contribute to much faster degradation of the barrel in the Throat, Leade and Muzzle areas. These are the primary controllers of accuracy so attention in this area is vital to both the consistency of any load and barrel life/accuracy.
  

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