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Normal Topic Heat vs Pressure vs Velocity – effects on accuracy, consistency and barrel wear - Part three (Read 655 times)
50BMG
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Heat vs Pressure vs Velocity – effects on accuracy, consistency and barrel wear - Part three
Oct 23rd, 2018 at 4:30am
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Having looked at some fairly interesting internal ballistic tech geek stuff, what does it all mean in the real world?

We are looking at the needs of the long-range shooter predominately here. More so the hunter from my perspective as I am simply not interested in shooting targets which behave with completely controlled predictably time after time under comfortable conditions. My daughter could hit 300mm square targets at 1200 metres all day long when she was 12 years old. It is not at all a challenge as far as I am concerned. I have made no secret that I have devoted a large part of my life to understanding and improving long-range shooting form a snipers perspective. The challenge of a target that can and does shoot back with a high degree of skill is what drives me. The next best thing to me is hunting animals at the limits of what I consider to be ethical and humanely responsible ranges. The goal here is a clean, quick, pain-free death and no wasted meat. I do not hunt trophies. Both scenarios have a large number of uncontrolled and unpredictable variables. So for those who may find what I say a little hard to fit their style of shooting I say this. The ideas, skills, techniques and methods I discuss will work exceptionally well for any type of shooting. You just need to swap the jargon for something which better aligns with you and then apply the information provided.

What can be done to reduce the effects of precession?
1.      Use the slowest powder that will work safely in the cartridge. This accomplishes several things.
A)      The slow burn means a peak pressure will occur after the projectile has entered the rifling. This reduces the media blasting effects at the Throat and Leade a lot. Tip, do not use a powder so slow that it is still burning after the projectile has left the muzzle. This results in significant uncontrolled turbulent expansion of gases around the projectile and destabilises it. It also increases “muzzle blast” potentially leading to flinching or other bad habits.
B)      A slow powder helps to counter the effects of atmospheric and rifle temperature variation and moisture content inside the loaded round. This reduces the variability of performance (in case pressure and temp plus projectile velocity) at different temperatures thereby providing improved consistency.
C)      A slower powder means less heat and in turn less increase in the malleability of the barrel steel and again less erosion.
D)      As the peak pressure is achieved sometime after the projectile has entered the rifling and not during the precession period, far less gas and particulate is forced into the microscopic grooves formed by previous firings (with far less heat and velocity) considerably reducing compounding erosion in those same grooves. This works with C above and your barrel will live far longer.
E)      A slower powder means the projectile is travelling with less speed and force when it makes contact with the rifling. This reduces the amount of or potential for the excessive engraving of the projectile and the subsequent accuracy degradation this causes.
F)      A slower powder means you can use good compressed loads.
2.      Use compressed loads but do not try this until you have had proper education on the technique. The difference between a powder burning as intended in the chamber and having the bolt blown back into your eye is sometimes 2 -3 grains once you start compressing loads. To successfully use compressed loads there is usually a need for increased neck tension to retain the projectile over the compressed powder. This requires a great deal of care and patience to get right. Again a little too much neck tension significantly raises peak pressure.
3.      The advantages of compressed loads are
A)      Consistency under varying atmospheric and rifle conditions.
B)      More uniform powder distribution in the case when lying horizontal leading to more uniform ignition and a burn performance. This means more accuracy and consistency.
C)      Reduced air and therefore moisture in the case. Same as B above
D)      Prevention of uneven pressure wave formation due to unoccupied case space. This means the projectile is not launched off centre
E)      D above leads to more concise alignment of the projectile with rifling reducing damage to barrel and projectile and improving consistency and accuracy,
F)      D also results in the less turbulent behaviour of expanding gases to upset the projectile as it leaves the muzzle.
G)      D results in more even barrel harmonics for improved accuracy and consistency.
Please be aware that the gains identified will vary according to the specifics of each rifle/ load combo. The degree of gain in each case is far from vast. In each case, the aim is to achieve a minimisation of accuracy degrading processes which, when added together, result in useful (and often impressive) accuracy and consistency gains.

As a final comment in this section, in our experience, the machining of the chamber and fit of the case into the chamber come into play here. Think about that round you have meticulously loaded, taking care to eradicate runout at all stages and eventually produce a round which has total concentricity.  When that round is loaded in the chamber and the bolt closed the round is actually pointing down fractionally due to the effects of gravity and the way the round is held by the bolt. This immediately creates an alignment issue. This misalignment does all but destroys the good work that can be achieved as outlined above. That means you need to have the chamber reamer and the related loading/ sizing dies made to allow for this phenomenon.
« Last Edit: Oct 26th, 2018 at 6:22am by 50BMG »  

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Re: Heat vs Pressure vs Velocity – effects on accuracy, consistency and barrel wear
Reply #1 - Oct 23rd, 2018 at 8:07am
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Interesting stuff.
I have a question.-- or two --

Some rifles are chambered with a long throat, say 10+ mm (400 thousands of an inch +) but most these days seems to have throats around 80 thou or even less.
Would you agree that in the case of a chamber having a very long throat, that such a chamber effectively increases in size as the bullet moves towards the rifling leade? --This can allow higher charge weights of a given powder (for a given bullet weight) than would be the case where jump would be very short.--

Would  a rifle with a very long throat tend to be more, or less, prone to throat erosion?
  

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Re: Heat vs Pressure vs Velocity – effects on accuracy, consistency and barrel wear
Reply #2 - Oct 26th, 2018 at 5:02am
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Huntfish

The combination of throat length, rifling taper and powder speed are what count not a single item. That said and being very general, the further the projectile has to travel to enter the rifling and seal the gap against precession the more erosion you will potentially get. Hotter and faster the gases and particulate mean more severe erosion with each shot.  There is a tendency to look at things in isolation and this does not help. it is essential to step back and look at what you are doing in a holistic, all-encompassing way for best grasp.
  

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