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Normal Topic Heat vs Pressure vs Velocity effects on barrel wear, consistency and accuracy. part two (Read 631 times)
50BMG
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Heat vs Pressure vs Velocity effects on barrel wear, consistency and accuracy. part two
Sep 10th, 2018 at 3:27pm
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So what actually happens when you squeeze a round off?

Most people will know that the primer ignites the powder which burns fairly quickly, driving the projectile down the barrel. How many know the acceleration of the projectile is around 400Gs?

Those of us who are keen on hitting targets a long way off tend to favour flat shooting, high-velocity loads pushing high BC projectiles. There is a fair bit of info available on how to achieve these loads and the potential they have for frequent investments in new barrels because of various types of wear. We are all cautioned to avoid overheating our barrels to try and extend the life of them.

For those who intend to hunt at long ranges the cold bore or first shot performance and accuracy are likely to be of most interest. For others ringing gongs or shooting competitively, repeater performance is more likely to be to the fore. In all cases, there seems to be a conflict between heat, pressure and velocity vs barrel wear, consistent performance in varying conditions and absolute accuracy.

Firstly to clear up a common misconception, modern propellant powders for small arms do not explode, they burn very, very quickly and this is important. The speed of burn is what allows fine control and safe use of firearms. Using true explosives as propellant would simply destroy the firearm and injure the firer.

There is a myriad of powders available. They vary from fast to slow burning (comparatively) and also in temperature, and all have a specific set of performance characteristics to provide a predictable internal ballistic result. There are temperature controlling, flash reducing and antifouling additives to name but a few.

While burning the propellant powder is the major component in terms of generating pressure to drive the projectile to the desired velocity there is another little known and very disruptive component added with every load – atmospheric moisture. If you load a box of ammo at 75% humidity and another at 50% humidity there will be nearly twice as much water in the first load as the second. Why is this important? There are several reasons.

The first is that moisture turns to water vapour (steam) at a mediated value or 1: 1600 approximately. This has implications in peak pressure achieved and time to peak pressure.

The second is that the speed at which the water is vaporised varies according to the temperature at which the round is ignited. This significantly impacts consistency of load performance across a range of temperatures. It will also make the ES appear erratic with temperature or pressure change

The third is that a higher moisture load will reduce load density, alter expansion ratio and increase precession.

The first two just make life a bit more difficult for the most part. The third contributes significantly to throat and leade erosion.

Studies we undertook showed beyond any doubt that a combination of very fast pressure rise and elevated moisture content in the load create significant precession. The effect of this is a form of media blasting of the throat and leade.

We found that by loading ammunition at or below 50% humidity had a remarkable effect in reducing measurable wear. The best way to ensure consistent low water content is to have all the loading equipment, tools and supplies in the same climate controlled room for at least 24 hours before commencing loading. In this way, you increase consistency from load to load by reducing the slight dimensional changes in the tooling etc. due to temperature variation but more importantly you reduce the microscopic water deposits inside the cases, tooling and even in the powder. We used settings of 22 Celsius and 50% humidity. Just doing this gave us between 200 and 350 more rounds per barrel with a 300RUM This was measured using 10 samples each of three different barrel brands. 5 at random loading conditions and five at controlled loading conditions. We also found the ES tended to reduce (10 round groups by 8 - 11%) and velocity was far more constant across a wider range of atmospheric pressures and temperatures. In other words, accuracy and consistency were significantly improved.

When combining the above with duplex loads results improved further. The reason for this is that a duplex load will give faster ignition time and more even burn but still allow a fully filled case (or even slightly compressed load). Fully filling a case means that the absolute minimum of air is included. Air carries moisture. As shown above, moisture is a wildcard and needs to be controlled.

There is a serious safety issue here. Do not try to create a duplex load unless you have access to the correct data and do so under expert guidance. Get this wrong and you will get hurt or hurt someone else.

In an ideal world, those of us looking to hit targets on or near the horizon would be using duplex loads at a medium to high compression. I can say that when I have approached powder suppliers with a request for this type of data they just refuse outright or deny it exists. If I use my contacts it is easy to get. Understandably they are concerned about being sued when some idiot does the predictable and then looks for someone else to blame.

So to get the best you can with what is available what you are looking for is a load that is carrying the least amount of moisture, has no airspace, ignites really quickly but builds pressure relatively slowly and therefore avoids excessive heat. This will set up an expansion ratio which makes the most use of the space and time available to give the most even results possible without eating barrels.

The next step is to consider velocity and ES. As most are aware velocity is a raw measurement and tells you very little other than the obvious. They are a great many people who will pursue an extra 100 Fps velocity because they perceive that little bit flatter trajectory will serve them better. Trouble is 100fps difference can just be a symptom of the firing conditions at the time. Secondly, that little extra bit of raw velocity is highly likely to be at reduced accuracy and accelerated erosion.

Typically ES is expressed as so many feet per second. This does provide some good and useful data to base decisions on. However, go back to my point above, it can be no more than a happy result due to very favourable conditions. When you measure ES as a percentage of total velocity the figures are often quite surprising. Let’s say you fire 10 rounds and record 3210, 3212, 3213, 3220, 3211, 3216, 3215, 3218, 3223, 3219. This gives a total velocity of 32157fps. In traditional terms, the ES would be 13 and not bad.  As a percentage, it is 0.0404%. Reduce those velocities (keeping all else the same) so they are 2210, 2212, 2213, 2220, 2211, 2216, 2215, 2218, 2223, 2219 still the ES is 13 but as a percentage, this is 0.0586% and therefore not as good as the first example. The amount of energy difference (powder burned) and therefore the potential for erosion due to precession is huge. The external ballistics will also be totally different. Clearly, the ES stated in Fps is not as useful as it might seem as a comparison.

To come back to the point to be made, it is very common to find you have two good loads with around an average of 100 – 150fps difference. Both show an ES of say 12 and give very similar grouping performance. Pick the one with the ES as the lowest percentage of total velocity. You can put your left ball on the fact it will have the slower total burn time, lower temperature and lower peak pressure. This is because the faster you build pressure the more erratically and hotter things get. Don’t burn so fast and the temperature is reduced. Reduced speed of pressure and temperature means less precession and less barrel erosion.

The moment you see your ES begin to increase as a percentage you are going the wrong way.
« Last Edit: Sep 11th, 2018 at 2:48pm by 50BMG »  

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Re: Heat vs Pressure vs Velocity effects on barrel wear, consistency and accuracy. part two
Reply #1 - Sep 11th, 2018 at 10:19am
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A question please,  as the guidance on ES appears to need some clarification:

"Pick the one with the ES as the lowest percentage of total velocity"

For a fixed ES across two or more loads  - such as 13fps as in the example -  The ES will always be the lowest percentage of total (or averaged) velocity for the fastest load.

However I get the impression you are indicating that if two good loads have been identified somewhere within the  50kPSIpeak-to-CIPlimit range, then the the lower of these is preferred for long term consistency and lower barrel wear.

So are you saying that if the ES is recorded for these two loads, that we would find that even if both are local minima, the ES will likely be better as a % for the lower load ?
« Last Edit: Sep 11th, 2018 at 9:08pm by puffin »  
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Re: Heat vs Pressure vs Velocity effects on barrel wear, consistency and accuracy. part two
Reply #2 - Sep 11th, 2018 at 2:45pm
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Puffin

I did wonder about wandering into this topic.

The key is to find the lowest ES as a percentage of total velocity

It is not uncommon to find that you have a situation where the total velocity for two different loads is only marginally different as a raw number but significantly different as a percentage of total velocity. In this case, the higher load will show better.

However, it is frequently possible to find a slower load (often 50 -100fps less) with a slightly larger ES (say 13 vs 11) will actually be the better load and then you are correct.

I now realise I need to edit the post to clarify it. Thanks for that.
  

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Re: Heat vs Pressure vs Velocity effects on barrel wear, consistency and accuracy. part two
Reply #3 - Sep 11th, 2018 at 10:16pm
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50BMG,

I'm very interested in this ES aspect of your post and appreciate your sharing your hard-earned insights, but so far the explanation  - and unfortunately your follow up post too - has me scratching my head sorry.

Without meaning to be presumptuous could I propose that we start with your comment that

"it is frequently possible to find a slower load (often 50 -100fps less) with a slightly larger ES (say 13 vs 11) will actually be the better load

and perhaps you could build an example around this to show under what situation this would be true ? 
   
  
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Re: Heat vs Pressure vs Velocity effects on barrel wear, consistency and accuracy. part two
Reply #4 - Sep 12th, 2018 at 8:08pm
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Puffin

I apologise for furthering the confusion. As I am far from a mathematical genius I will make contact with the two Genii who can assist me to clarify this.

I can say that as an individual frequently tasked with applying a pretty decent ability with a rifle to ensure the safety of a group of others in the most volatile and violent shitholes on the planet I have been very keen for any edge to be found. I was not hugely interested in the finer detail rather I wanted a basic overview and a demonstration in realistic conditions proving it.

Once I had applied it and proven its value it was adopted into the program for deployment across the operational and training aspects of my work.

As far as this particular topic goes it was well and truly bourne out.

Please bear with me while I try to get your question answered by those who are best equipped.  Undecided Undecided
  

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Re: Heat vs Pressure vs Velocity effects on barrel wear, consistency and accuracy. part two
Reply #5 - Sep 12th, 2018 at 9:02pm
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Appreciated !

By the way,  the moisture content findings were fascinating. As you point out it is not everyone who can commit 30 barrels and 20-30,000 rounds to discover this sort of thing.
  
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Re: Heat vs Pressure vs Velocity effects on barrel wear, consistency and accuracy. part two
Reply #6 - Sep 13th, 2018 at 2:07pm
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That was the best part of not having budget or equipment restrictions Cheesy


The downside is a trigger finger calluses like you would not believe!
  

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Re: Heat vs Pressure vs Velocity effects on barrel wear, consistency and accuracy. part two
Reply #7 - Sep 15th, 2018 at 5:53pm
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Puffin

Sorry for the time taken. I have been soundly castigated and having realised my mistake hang my head in shame. My head was so full of where I wanted to go I forgot to clarify the start point. In so doing I muddied the waters.  What I forgot to do was set the qualifying criteria. So I will do so here. I will also flesh out my post somewhat.

The rule that a load delivering acceptable ES being the lowest percentage of total velocity produced (add together the velocity of all rounds in the group) will always indicate the least destructive (to barrels) for accuracy load. Developing increased velocity will always be at the expense of barrel life as a general rule.

It must be remembered that the greatest barrel life is to be had by finding a suitable load with the least amount of testing. What is often missed by the use of small groups is the ability to predict load performance across the entire range of likely conditions in which that load will likely be used.  So the larger the group the less disparity will be found by this method. However, with small groups (inevitably containing higher error ratios), this process becomes extremely efficient in sorting the useful from the less useful.

1.      As velocity increases so does the energy produced to produce it.
2.      The extra energy is produced in the same vessel and therefore the violence increases exponentially.
3.      Increasing violence generally has an equal increase in volatility of results in all but a few “sweet spots”
4.      Because of the cost of barrels and materials used to manufacture ammunition many people use three or five round groups to try and gain an indication of likely performance of any given load.
5.      This method is particularly useful for comparing loads where the projectiles are of different materials but similar weights as the internal ballistics will be significantly different and precession (degree of) is going to be a major factor on projectile stability from case to rifling and exiting the muzzle ultimately affecting accuracy.
6.      As loads become more compressed this method will become less useful.
7.      This method is particularly useful as it will show you when a load with a significantly higher raw ES is actually the more accurate load.
8.      A load with a higher velocity will usually prove the more accurate all else being equal.


To your question – my statement somewhat clumsily stated"it is frequently possible to find a slower load (often 50 -100fps less) with a slightly larger ES (say 13 vs 11) will actually be the better load" without clarifying it is in reference to 3 round groups. By the time you get to 10 round groups the data quality is improved to the state that your observation - “The ES will always be the lowest percentage of total (or averaged) velocity for the fastest load.” Holds true except in the rarest of circumstances.

I apologise for the lack of initial clarity.
  

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Re: Heat vs Pressure vs Velocity effects on barrel wear, consistency and accuracy. part two
Reply #8 - Sep 17th, 2018 at 9:15am
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Thank you.
  
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