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barrel length and powder burn

nemesisenforcer
we all know that longer barrels burn more powder, but in what way to do shorter barrels burn powder? In other words, if I load a case with 50 grains of powder that is substantially all burnt in a long barrel but only 40 grains (80%) is burned in a short barrel, if I up the charge to 55 grains, will only the 40 grains be used or will it be 44 grains (80%?)

Does more powder in a case equal more powder burned in the shorter barrel (but still less than in an optimal length barrel) or are you wasting that much more fuel and reach a point of diminishing returns on an absolute level?
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  • 5mmgunguy
    A very interesting question, and not a simple one to answer. Because:

    You have more variables than just barrel length and case capacity,you need to consider such things as the burn rate of the powder too.

    Also case to bore capacity needs to be considered to.
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  • jaegermister
    I suggest you read a portion of The Bullet's Flight from Powder to Target, by Dr Mann available on line/free google books. The chapters about barrel length and smokeless powder, your question is so open that it requires considerable understanding of what happens as smokeless/progressive powder burns. This book will probably answer any further questions you may have. Dr Mann tried everything you can imagine to understand the topic.
    In general I have found that by using a chronograph and placing a white cardboard or sheet under and in front of the muzzle I am able to determine the optimum powder/type/volume for the load. When unburnt powder appears on the cardboard and velocity does not increase and remain "constant" I am exceeding optimum powder type/volume/barrel length, all the while staying within the pressure levels required.
    My findings appear that 15 to 18 in of barrel is needed for most rifle powders, less than that and you need a pistol powder, any longer does not effect velocity much. Case capacity needs to be just under full but not compressed for best results. Try posting this topic over in the reloading forum
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  • Tailgunner1954
    The suggestion about reading Dr Mann is a good one, for all shooters. It's deep but well written in the style of his era. Today's ballistic "experts" all start with Dr Mann's work and try to expand on it. IOW Dr Mann is to shooting as Issac Newton is to physics

    That said, and assuming the same cartridge, the longer barrels give more TIME for the powder gas to work on the bullet resulting in higher velocity. All the powder that's going to be "burned" will be converted to gas by the time the bullet is 6" down the barrel (in the case of some pistol rounds, it's all converted before the bullet leaves the case).
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  • nemesisenforcer
    quote:Originally posted by Tailgunner1954
    The suggestion about reading Dr Mann is a good one, for all shooters. It's deep but well written in the style of his era. Today's ballistic "experts" all start with Dr Mann's work and try to expand on it. IOW Dr Mann is to shooting as Issac Newton is to physics

    That said, and assuming the same cartridge, the longer barrels give more TIME for the powder gas to work on the bullet resulting in higher velocity. All the powder that's going to be "burned" will be converted to gas by the time the bullet is 6" down the barrel (in the case of some pistol rounds, it's all converted before the bullet leaves the case).



    then why does a larger charge of powder in the short barrel produce more velocity than a smaller charge? Just more gas period?
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  • armilite
    When I use a shorter barrel I tend to go with a powder that is faster burning, but a powder that works within that calibers parameters. With a longer barrel I would go with a slower burning powder within the parameters of that caliber as well. Even if I had two .308's one with a 16" and the other with a 26" barrel I'd be likely to experiment and possibly use 2 different powders for the same caliber. There's more to a good load besides sheer velocity. In regards to your original post it isn't the length of the barrel so much as its more about the powder's burn rate.
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  • MIKE WISKEY
    "then why does a larger charge of powder in the short barrel produce more velocity than a smaller charge? Just more gas period?'........The burn RATE is also affected by pressure so more powder=more pressure=more velocity
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  • beantownshootah
    The exact numbers and percentages are going to vary on a case by case basis depending on what powder you're using, primer, temperature, bullet weight, bore cross-sectional area/volume, crimp, and other factors.

    But in general, yes, all else being equal more powder in a cartridge with a short barrel DOES yield more velocity (eg .357 magnums are still quite a bit more powerful from 2" bbls than .38s).

    By extension, a higher muzzle velocity MUST mean a greater MASS of powder burned inside the barrel (ie grains burned), if not necessarily a greater percentage of the overall powder charge loaded.

    In fact, efficiency (ie percentage burned) is probably going to go DOWN with increasing mass of powder, but thats subject to a whole slew of variables, as above, and I don't think you can absolutely generalize this.

    I think the main limiting factor to max velocity from a short barrel is going to be how much powder you can stuff in the cartridge and still get efficiently lit with the primer.
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