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Thread: Pressure vs velocity

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    Default Pressure vs velocity

    Alrighty fella's, I got something stuck in my head and can't wrap my brain around it.
    Why is it you can see a list of different powders for a specific bullet have a wide range of pressure yet roughly the same velocity? I keep thinking a pressure increase should propell the bullet faster????

    And, by going from a std primer to a mag primer should one expect a pressure increase or velocity increase, neither or both????

    Perhaps its time to go to the woods and just wonder around behind the critters!
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    The bullet is accelerated by pressure over time. When pressure is discussed it is the peak pressure that is referenced, but what is important is the are under the time/pressure curve. It is the area of the curve that accelerates the bullets, not the peak pressure. A Faster powder will reach peak pressure sooner, but it's pressure will also decay faster which is why it will produce a lower velocity than a slower powder.
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    Quote Originally Posted by Paul H View Post
    The bullet is accelerated by pressure over time. When pressure is discussed it is the peak pressure that is referenced, but what is important is the are under the time/pressure curve. It is the area of the curve that accelerates the bullets, not the peak pressure. A Faster powder will reach peak pressure sooner, but it's pressure will also decay faster which is why it will produce a lower velocity than a slower powder.
    Great explanation Paul - thx I think I am with ya.
    So, by jumping to a magnum primer it could be possible to cause a decrease in velocity because it will generally make a powder ignite quicker or at least more uniformally - which could cause a fast burning powder to peak faster thus a probable reduction in velocity?
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    Quote Originally Posted by Paul H View Post
    The bullet is accelerated by pressure over time. When pressure is discussed it is the peak pressure that is referenced, but what is important is the are under the time/pressure curve. It is the area of the curve that accelerates the bullets, not the peak pressure. A Faster powder will reach peak pressure sooner, but it's pressure will also decay faster which is why it will produce a lower velocity than a slower powder.
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    Quote Originally Posted by Smokey View Post
    Great explanation Paul - thx I think I am with ya.
    So, by jumping to a magnum primer it could be possible to cause a decrease in velocity because it will generally make a powder ignite quicker or at least more uniformally - which could cause a fast burning powder to peak faster thus a probable reduction in velocity?
    Well yes thats possible but by going to a hotter primer more likely youll get more peak pressure but still have similar velocity. Thats because the pressure/time curve shape may change but the powder will still produce X volume of gas. Same volume of gas will likely make the overall average of the pressure/time curve very similar . . . so you should still net a similar velocity with the same amount of the same powder. Hope that don't muddy back up the waters Paul filtered for ya.


    There are so many possible primer/powder combinations that odd things like that do happen. However if you are seeing this velocity drop from magnum primers my first suspicion is you went to a colder primer and just thought it was hotter because the box said magnum. Id suspect you went from a brand X primer to a brand Y magnum primer and X primer was hotter even though its not magnum. Some brands of primer are just hotter, some so hot that a normal primer in one brand can be quite a lot hotter then even magnum primers of another brand.
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    I have seen primer experiments written up where changing primers (from mild to hottest) can add as much as 4,000 psi. This large change usually occurs in small capacity cases, and primarily in straight-walled pistol cartridges. In larger rifle cases pressure increases will be much less, around 1,000 psi or less.

    Hotter primers can also increase velocity ES and group size, if not really needed for consistent ignition. With larger capacity cases and hard to ignite slower powders, ES and group size can shrink with hotter primers. Use them only if beneficial or necessary.

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    Quote Originally Posted by drow View Post
    I have seen primer experiments written up where changing primers (from mild to hottest) can add as much as 4,000 psi. This large change usually occurs in small capacity cases, and primarily in straight-walled pistol cartridges. In larger rifle cases pressure increases will be much less, around 1,000 psi or less.

    Hotter primers can also increase velocity ES and group size, if not really needed for consistent ignition. With larger capacity cases and hard to ignite slower powders, ES and group size can shrink with hotter primers. Use them only if beneficial or necessary.
    Primer choice definitely effects ES and accuracy. So does bullet pull tension in the same way, a bullet that stays a bit longer due to higher pull tension gives the primer more time to light the powder.
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    Way ta go Smokey! Now look what ya started! Don't be askin nothing in relation to barrel length or this thread will never get over.

    There is some stuff we just don't need to know!

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    Quote Originally Posted by elmerkeithclone View Post
    Way ta go Smokey! Now look what ya started! Don't be askin nothing in relation to barrel length or this thread will never get over.

    There is some stuff we just don't need to know!
    Shucks EKC, I wuz a pondering if pressure readings change when barametric pressures fluctuate but hunting season has arrived and between shooting doves, geeses, and catching a bucket full of slab bluegills and stripers this past weekend I am finding little time for such brain stimulators!
    I do appreciate the inputs fella's and its helped me better understand some of my thoughts, so thanks to all the reply's!
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    Quote Originally Posted by Paul H View Post
    The bullet is accelerated by pressure over time. When pressure is discussed it is the peak pressure that is referenced, but what is important is the are under the time/pressure curve. It is the area of the curve that accelerates the bullets, not the peak pressure. A Faster powder will reach peak pressure sooner, but it's pressure will also decay faster which is why it will produce a lower velocity than a slower powder.
    That is a good explanation.

    Consider the fast powder in this example finishing it's burn/push and producing a spike in peak pressure before the bullet leaves the barrel. That bullet will have a lower velocity because the remaining bore length is dragging down the velocity, no more burn, no more push. It's like hitting the brakes.

    A slow burning powder will have a lower pressure and velocity since it's push hasn't peaked prior to the bullet leaving the barrel. This continued burn will cause muzzle flash.

    The optimum powder burn will peak at the exact time the bullet leaves the crown. This will give the best velocity at an equivalent pressure value when compared to the fast burn powder.

    A powder that is very slow burning will not approach the max pressure value for that application. Imagine a 308 Win case with a full charge of Retumbo. It will shoot just fine but it will be slow and it will flash like crazy. The only way to make it work would be a custom 30" barrel. The optimized powder in the shorter barrel would create higher pressure in the long barrel but the velocity would be slower.

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    Quote Originally Posted by marshall View Post
    That is a good explanation.

    Consider the fast powder in this example finishing it's burn/push and producing a spike in peak pressure before the bullet leaves the barrel. That bullet will have a lower velocity because the remaining bore length is dragging down the velocity, no more burn, no more push. It's like hitting the brakes.

    A slow burning powder will have a lower pressure and velocity since it's push hasn't peaked prior to the bullet leaving the barrel. This continued burn will cause muzzle flash.

    The optimum powder burn will peak at the exact time the bullet leaves the crown. This will give the best velocity at an equivalent pressure value when compared to the fast burn powder.

    A powder that is very slow burning will not approach the max pressure value for that application. Imagine a 308 Win case with a full charge of Retumbo. It will shoot just fine but it will be slow and it will flash like crazy. The only way to make it work would be a custom 30" barrel. The optimized powder in the shorter barrel would create higher pressure in the long barrel but the velocity would be slower.
    I know a guy that shoots H4831 in all his guns including a short barreled 308. He's fine with his 243 and 7mm mag but his short barreled Mohawk 308 really makes the fire fly especially at dark.

    Really guys I was just funning Smokey! There has been a whole lot of good said in this short thread!

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    Marshall,
    Thats probably the best explanation I have seen - well done!
    I tried to tell ya EKC that you wuz cut'n that 358 bbl too short - you'll be lucky if'n the bullet doesn't either get stuck in the bbl or simply drop out the end and land on your foot...
    Is there a chart of some type that shows powder burn distances????
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    Quote Originally Posted by Smokey View Post
    Is there a chart of some type that shows powder burn distances????
    There are many burn rate charts out there, here is one of the better ones.
    http://www.reloadbench.com/burn.html
    Nothing that shows distances that I know of. Its not so much distances (ie barrel length) that matters as total volume of the chamber and barrel. Then say 300WM and a 45-70, they may have similar (guessing) total volumes but its in different places, they are pushing very different bullets at different pressures so the optimal powder burn rate for a 20 barrel will be very different for each.
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    Wow! This is a very informative thread. I don't know what else I could add. Paul and others have summed it up. Good job guys.
    It is truly the area under the curve that does the work. The work here accelerating a mass down the tube.
    P vs v is what we attempt to manage by selection the correct powder burn rate for given bore size, bullet weight and expansion ratio. The faster the burn rate the quicker the peak pressure rise. The larger the bore the faster the peak pressure decay. Heavier bullets have more at rest inertia and accelerate more slowly and generally respond best with slower powders.
    The issue is further complicated by so many folks using combinations that defy thes principles but still work. Still work in that the bullet leaves the barrel and the gun stayed together. Hand loaders should strive to optimize loads by understanding these principles. The reward is generally better accuracy and optimum velocity for the cartridge.
    fortunately the loading manuals do a pretty good job of getting us started in the right place.
    Ok. I guess I did find more to say.
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    Very good thread. I think I understand all that has been said here to this point with one possible exception. I was told once by one of the more knowledgeable guys over on another forum that for any given cartridge, the powder that gives the best velocity at one barrel length will give the best velocity at any other barrel length.

    The example we were discussing at the time was my .358 Norma. Most all of the manual data suggests the best performance will come from a powder in the 4350 range and this data is usually shown from a 26" barrel. My barrel is 22" however and 4350 does not come close to giving me the same performance as 4895.

    I always presumed this was because the faster 4895 developed its peak pressure more optimally in a 22" tube whereas the 4350 is still burning after the bullet is already out of the barrel and thus not ever developing its full potential. Am I off in this thinking?

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    Quote Originally Posted by evandailey View Post
    Very good thread. I think I understand all that has been said here to this point with one possible exception. I was told once by one of the more knowledgeable guys over on another forum that for any given cartridge, the powder that gives the best velocity at one barrel length will give the best velocity at any other barrel length.

    The example we were discussing at the time was my .358 Norma. Most all of the manual data suggests the best performance will come from a powder in the 4350 range and this data is usually shown from a 26" barrel. My barrel is 22" however and 4350 does not come close to giving me the same performance as 4895.

    I always presumed this was because the faster 4895 developed its peak pressure more optimally in a 22" tube whereas the 4350 is still burning after the bullet is already out of the barrel and thus not ever developing its full potential. Am I off in this thinking?

     
     
     
    No I dont think your too off but I dont think your knowledgeable guy was far off ether. Usually your best powder is going to be best within any reasonable barrel length changes, usually but not always though. Very little in reloading is always true due to the never ending array of little things that change other things. In this case it sounds like nether the 4350 (too slow) or 4895 (too fast) are the best but something between like maybe Varget may be closer to optimum for that caliber with whatever bullet you were using. If your choices are too fast or too slow fast will usually give better velocity and less flash because the slow is finishing too late, after the bullet is gone.

    Its not the peak pressure though its the total of the pressure for the time it has to act. I wish I could draw a couple charts for you but I aint that computer savvy. With a fast powder the line gets to peak faster then falls off faster making a spike peak that looks like an inverted V on a chart. A slower powder gets to the peak slower and falls slower to look more like an inverted U on a chart. Like Murphy says its the part under the line that does the work, the rounded top of the upside-down U means there is more total work doing pressure under the line. They both start at zero and last the same time but the slower one stayed nearer peak for a lot longer so has a much higher average or total pressure even though they both have the same peak pressure. Its that average of all the pressure that moves the bullet, all that pressure under the line not just the peak.
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    Quote Originally Posted by ADfields View Post
     
     
     
    No I dont think your too off but I dont think your knowledgeable guy was far off ether. Usually your best powder is going to be best within any reasonable barrel length changes, usually but not always though. Very little in reloading is always true due to the never ending array of little things that change other things. In this case it sounds like nether the 4350 (too slow) or 4895 (too fast) are the best but something between like maybe Varget may be closer to optimum for that caliber with whatever bullet you were using. If your choices are too fast or too slow fast will usually give better velocity and less flash because the slow is finishing too late, after the bullet is gone.

    Its not the peak pressure though its the total of the pressure for the time it has to act. I wish I could draw a couple charts for you but I aint that computer savvy. With a fast powder the line gets to peak faster then falls off faster making a spike peak that looks like an inverted V on a chart. A slower powder gets to the peak slower and falls slower to look more like an inverted U on a chart. Like Murphy says its the part under the line that does the work, the rounded top of the upside-down U means there is more total work doing pressure under the line. They both start at zero and last the same time but the slower one stayed nearer peak for a lot longer so has a much higher average or total pressure even though they both have the same peak pressure. Its that average of all the pressure that moves the bullet, all that pressure under the line not just the peak.

    Thanks for the reply Andy. That's all basically what I thought. I have no problem visualizing the pressure curves so no worries on the computer savvy.

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    I think there may be some misconceptions about powder burn so I think I'll wade in here again.
    First I would say that 4895 would be a much better choice for the 358 Norma than 4350 in any barrel length.

    Powder burn; All powder is burned, all that is going to burn, somewhere between chamber and muzzle. More on that later but pressure peak does occur a few inches down a rifle barrel. The faster (rifle) propellants produce a pressure peak closer to the chamber than the slower propellants but it's always a few inches from the throat of the chamber. The decay of the pressure curve is much slower than the rise and it is the slower decay if the slower burning powder that can perform more work. (a broad curve gives more area under the curve) Pressure is still several thousand psi at bullet exit. Consider the M1 Garand rifle that requires a port pressure of approximately 14,500psi. This port is about one inch from exit. This is achieved with 4895 powder. 4831 powder will raise that level to about 22,000 psi and can bend the op rod. If, as the bullet moves down the barrel it exposes enough "space", the pressure drops more quickly.
    This is determined by; 1. The volume of the swept bore. 2. The velocity of bullet travel. And 3. The energy of the propellant.
    Energy of the propellant is my way of saying, how much gas volume can the powder can continue to produce as more bore volume is exposed.
    Fast propellants are able to release their energy more quickly and can continue to maintain pressure in larger bore volume cartridges (45-70) They would cause an excess peak pressure, because of their rapid release, in a small bore volume cartridge. (264 WM)
    A propellant that can release lots of energy over longer time is more desirable in small bore volume cartridges and is considered the ideal for many. Higher energy powders such as those with higher percentages of nitro glycerine would not be so good if we could not control that release. Deterrent coatings and special shaping and perforations help with this.

    Now back to powder burn. Propellant powders are designed to burn , or more accurately, release their energy (this energy is in the form of heat and pressure, we use the pressure to push the bullet, the heat to change the POI of the barrel) at a certain range of pressures. The rate of release will vary from maximum to minimum within the confines of a few inches of barrel. A point down the barrel we say that powder is "all burned" even though it's still very much unburied. It's just released about 95% of the energy that it can release given that powder "burn rate", powder volume, bore volume, which includes bore length, diameter and bullet weight (because this effects velocity of travel and rate of bore volume exposure). There is always some powder left unbuned simply because the confinement in the barrel was not ideal for it to continue to burn and release oxygen which helps the burn, etc. given the above mentioned points. Still very hot powder exits every barrel and glows anew with the new found oxygen at exit.
    Obviously some powders will exit with more unburned kernels than another burn rate powder.
    Consider 4198 (45-70) vs 4831 (264WM.
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    That works Murphy, you are correct as usual. I was just leavening a lot of that out for now trying to get the big points to make sense. Its a lot easer to splain it all correctly visually than with the confusion of type alone.
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    OK, that does it - I am selling all my reloading equipment and sticking with factory loads! Does anybody eles have a headache?
    Just kidding - lots of interesting stuff fella's def for the advanced reloader class!
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