Calculating muzzle velocity from powder burn rates

Calculating muzzle velocity from powder burn rates

This is a discussion on Calculating muzzle velocity from powder burn rates within the Reloading forums, part of the Defensive Ammunition & Ballistics category; Can anyone come up with a burn rate for a powder? Any powder really. I looked at the formula that was created by Powley( POWLEY.HTM~ ...

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Thread: Calculating muzzle velocity from powder burn rates

  1. #1
    New Member Array mgondek43's Avatar
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    Calculating muzzle velocity from powder burn rates

    Can anyone come up with a burn rate for a powder? Any powder really. I looked at the formula that was created by Powley(POWLEY.HTM~) but that assumes that the psi is a constant for the equation unless I have read it wrong. I am currently trying to write a program that does something very similar to the slide calculator mentioned for my engineering class but I have been running into roadblocks everywhere trying to calculate a crude muzzle velocity given the bullet weight, caliber, grain. Is this impossible? Any suggestions on other ideas to create a simple program with nearly the same topic? Any and all advice is greatly appreciated!!


  2. #2
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    The short answer is no. Burn rate is highly dependent on grain size and shape (flake, ball, cylindrical, etc.), compositiion, of course, and then how densely packed. Quoting from the NRA Firearms Fact Book, "while... one grain at a time burns very slowly, when several grains or a pile of them are ignited a very hot and extremely quick fire results..." The volume and shape of the cartridge also affect the burn rate, even for a given powder, as does the diameter of the bore.

    Powley's equation uses a constant pressure in order to come up with a fairly useful result. In reality, the pressure behind the bullet while it remains in the barrel varies with time, and different powders with their different burn rates require different barrel lengths to achieve efficient use of the powder's potential. One cartridge, bullet weight and powder combination may result in the peak pressure occurring barely after the bullet has left the cartridge case, while another combination reaches peak pressure with the bullet halfway down the barrel. And just to make things fuzzier, both loads may impart the same exit velocity (or even muzzle energy, if different bullet weights) to the bullet.

    My simplistic approach, if this was something I either had to do or had ample time to do, would be to grab a ballistics program for handloaders and develop a family of curves for a given caliber with maybe just two variables, such as powder charge and bullet weight. Or find someone else's data. Then work backwards using something like TK Solver or MathCaD to curve fit the results. It'll be empirical rather than scientific, but it might give you a useable result.
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    VIP Member Array high pockets's Avatar
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    All I have ever seen are charts showing relative burn rates. I just looked again and none of them seem to give an actual burn rate.
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    VIP Member Array glockman10mm's Avatar
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    I'm not a scientist, or have any thing hanging on my wall that says I even have a brain, but, I've been handloading for slot of years, and I can't see how it could be done.

    The things gasmitty pointed out are spot on. Then, theres the issue of barrel length; does the powder reach it's peak burn before the bullet leaves the cylinder? Or, is it fired from an auto or single shot vs a revolver? The jump from the cylinder into the forcing cone in a revolver can have variances with each round fired.

    I guess that's why we still have reloading manuals that even cannot give absolutes, and vary from publisher to publisher.
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    VIP Member Array ccw9mm's Avatar
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    Burn rate implies an amount of gasses produced over some time. If the powder makers could come up with a volume-of-gasses-produced-per-unit-of-time type standardized measurement for every powder produced, then I think that "number" could be used to help estimate muzzle velocity. For example, a given powder (of whatever shape, density, type) resulted in a "550" measurement, whereas another completely different powder resulted in a "300" measurement (less gas over time).

    Until then ...
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    Great post gasmitty. Correct me if I'm wrong too guys, (you've definitely reloaded more than I have) but I think a good example of the complexity of what the above posts are saying is when you look at the ratios of powder charges for the same bullet and I assume, through the same firearm they use to obtain the FPS results and maximum pressure loads by types of powder.

    For example: Referring to my handy dandy 7th Edition Hornady Handbook, I'm looking at .38 spl. showing a 140 grain LSWC and I will use the two extremes in the max pressure field, each yielding a significantly different velocity.

    Powder - TITEGROUP max recommended powder charge is 4.7 gr., yielding 950 fps.
    TRAIL BOSS max recommended powder charge is 3.8 gr., yielding 750 fps.

    There are 6 choices for powder charges for TITEGROUP. (700, 750, 800, 850, 900 and 950 fps)
    There are 2 choices for powder charges for TRAIL BOSS. (700 and 750 fps)

    So, if there were to be a uniform constant across different powders, you should be able to charge say, 4.1 grains of TRAIL BOSS to maybe get to 800 fps right? No, they stop at a max of 3.8 grains due to pressure maximums for its particular rate of burn throughout its cycle.

    Now my guess here in this case is that TRAIL BOSS has a higher pressure clime rate to burn time ratio than TITEGROUP even though it takes TITEGROUP .5 grains less of a charge, (3.3 gr.) to achieve the same velocity, (750 fps) as TRAIL BOSS does at 3.8 grains. Maybe some of the more knowledgeable folk here could clarify that for me if I'm on the wrong track there.

    For sure it's easier just to look at the book to see what I'm talking about but when you do and start to compare powder charges and velocities, it will tend to get you thinking, "Why?"

    All of this tells me one thing; I will never grab a random powder and attempt to guess how much it will take without standing on the shoulders of giants before me and refer to a reputable manual. Even at that, I tend to make loads at least one powder charge under the max pressure recommendation.
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    If you put a small pile of any powder in the palm of your hand and light it ,the smell of your burnt flesh will reach your nose about the same time as your vocal cords emit a girly scream
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    VIP Member Array mprp's Avatar
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    duk, is that a past experience? I guess there could be a correlation between the time it takes someone to scream when lit to the velocity you will achieve when it's loaded in a cartridge. Would make for an interesting chart anyway.
    Vietnam Vets, WELCOME HOME

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    Powder that has been stored properly should burn at the same rate under the same conditions. So you could time it - and then you'd have your burn rate. Call it feet per second, at sea level, at 70 degrees, 30 percent humidity, barometric pressure, moon phase, blah blah blah. Now you know how fast your powder burns in feet per second.

    Now you take that powder - and you put it in a cartridge behind a bullet.
    How much case volume is taken up by the bullet? How thick is the case? How tight is the chamber?
    What is the freebore/leade?
    What is the barrel length?
    Does the cartridge headspace off the breech or the shoulder or something else?
    How much difference is there between the front of the case and the front of the chamber?
    How heavy is the bullet?
    How many grooves in the rifling?
    How deep are the grooves - and how wide?
    How sharp are the edges of the grooves?
    What is the friction coefficient (if there was such a thing) for that particular weight and diameter and design and manufacture of bullet in that particular barrel?
    Is it still 70 degrees outside (and inside the chamber, and inside the cartridge)?
    Is the cartridge waterproofed?
    Was it 30 percent humidity when it was waterproofed?
    Is the brass itself porous?

    .....and so on.

    There are many factors that effect the burn rate of any powder. Temperature, Pressure, Volume - those are big ones. But every little variable will have an affect on velocity. These effects will be different for every powder - some powders will burn slower when warm, others will burn slower when cold. Almost every powder will burn faster when there is more pressure and less volume - but the amount of effort required to calculate the velocity based on those factors would be completely unreasonable - and still not 100% accurate. Every chamber is a little different. Every barrel is different. Every piece of brass, every primer, every flash hole....Even the individual flakes or balls of powder are all different. It is not an exact science. With a lot of work and effort, very consistent velocities can be achieved in a particular gun with a particular load.....or so I'm told. I'm not a benchrest shooter. I like the science behind reloading, and I like to shoot.

    I got this model of chronograph about 6 years ago. I don't remember how much it cost back then....but it's on sale now! Shooting Chrony F1 Chronograph

    Happy reloading to you!

    Austin
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  10. #10
    VIP Member Array aus71383's Avatar
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    ....and also - well said mprp. There isn't always a rhyme and reason that makes simple logical sense on the surface - it is complicated.

    Austin

  11. #11
    VIP Member Array aus71383's Avatar
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    ...and ccw9mm - the volume of gas produced by the weight of powder would be relatively easy to calculate. The issue arises with the speed at which those gasses are produced. Too slow and you have a squib, too fast and you have a mechanical detonation. Different chambers, different barrels, different headspaces....it just can't be done. It's a nice thought, but it would already exist if it were such a good idea. The reason we have load books and chronographs is so that we can know what is safe and what is accurate.

    Austin

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