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Thread: Powder Burn Rate and Barrel Length

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    Member Armymark's Avatar
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    Default Powder Burn Rate and Barrel Length

    Ok you ballistic sages, edgamicate me. Is there any relation between powder burn rates and the length of the barrel? It is obvious that slower burning powders are for longer barrels. This is evident by the difference between pistol and rifle powders. But when you get to many of the rifle powders, barrel length seems to have little influence on the use of specific burn rates. Is it the weight of the bullet and its resistance to initial movement? If you have a .450 Marlin in 24inch barrel and .300 Win Mag in the same length, why the difference in powder burn rate? If so, it would seem to me that you would want to build pressure behind the heavier bullet slower than the lighter bullet, yet this is not the case.

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    I’ll jist take a stab at this, and see what the experts have to say.

    It has to do with something called “expansion ratio”, for those who use the term, taking into consideration case capacity, bore diameter/caliber, as well as barrel length. How restricted the pressure is, I guess.

    Slower rifle powders are more suitable (give higher velocity) for big cases, and/or heavier bullets. I think regardless of barrel length. (within reason)

    Howsomever, there may be reasons to use a faster powder.

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    Math out the total volume of the case and bore for both guns you listed and you see the 450 has a much greater volume. Now that volume must be filled with gas and with a larger bore you need faster powder to keep pushing at an optimal rate. With say a 22-250 you need a slower burn so you match the smaller volume of the 22 bore and not get a pressure spike since the bullet can only move so fast down the pipe. Same is true of shotguns, they use powders with burn rates like a handgun due to all the volume that needs filled as the wad moves down the bore.
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    Quote Originally Posted by ADfields View Post
    Math out the total volume of the case and bore for both guns you listed and you see the 450 has a much greater volume. Now that volume must be filled with gas and with a larger bore you need faster powder to keep pushing at an optimal rate. With say a 22-250 you need a slower burn so you match the smaller volume of the 22 bore and not get a pressure spike since the bullet can only move so fast down the pipe. Same is true of shotguns, they use powders with burn rates like a handgun due to all the volume that needs filled as the wad moves down the bore.
    Let my little brain grab it... The burn rate is not necessarily a time rate, it is a volume expansion rate. So 50 grains of H1000 does not burn up slower than 50 grains of H322. H322 fills the void behind the bullet faster than H1000 would in the same amount of space. This rate of expansion allows us to control pressure. If the gas expands too fast, we have a pressure spike, too slow and we lose velocity.

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    Quote Originally Posted by Armymark View Post
    Let my little brain grab it... The burn rate is not necessarily a time rate, it is a volume expansion rate. So 50 grains of H1000 does not burn up slower than 50 grains of H322. H322 fills the void behind the bullet faster than H1000 would in the same amount of space. This rate of expansion allows us to control pressure. If the gas expands too fast, we have a pressure spike, too slow and we lose velocity.
    Time is a factor, a faster powder makes "X" volume of gas in less ‘time’ than the slower powder will. So it's how fast it expands to make the gas rather than how fast it burns up that matters to us. That’s it though, you got it . . . that brain isn’t so small after all.
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    So if I get what your saying Andy...

    Hypothetically reading between the lines. If you take your favorite rifle, one that you have carefully optimized with hand loads and you cut a few inches off your barrel you would reduce it's volume. That would require a slower powder to optimize it's performance, is that correct?

    It seems the opposite to me since you have less time to get that bullet up to speed in my example. I guess I need to do some research in this area.

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    Cartridge dimentions have a bit in this as well. That 45-70 or any straight walled cartridge tend to need a fast burn rate as well where the greater the bottle neck the slower the powder. This is just in general and is very much dependent on the factors which have already been stated. They go hand in hand. But for any given cartridge if you have shorter barrel a powder with in the cartridges given range of usable powders with a faster burn (in that cartridge) can give better performance where if you had the same rifle with a longer barrel you could use a slower powder. And this not even considering factors of different bullet weight.

    Example a 270win in a 20" carbine H4350 may be best but with a 24" barrel 4891 or even slower might be better.

    Just a another way to look at it to get your thoughs around the concept one with seemingly infinent variables.

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    Quote Originally Posted by marshall View Post
    So if I get what your saying Andy...

    Hypothetically reading between the lines. If you take your favorite rifle, one that you have carefully optimized with hand loads and you cut a few inches off your barrel you would reduce it's volume. That would require a slower powder to optimize it's performance, is that correct?

    It seems the opposite to me since you have less time to get that bullet up to speed in my example. I guess I need to do some research in this area.
    Loping off length does not change the bore size or time it takes the bullet to be at the same point in the bore. So a shorter barrel still takes the same burn rate due to the time factor, but less of the powder charge will be taken advantage of before bullet exit.

    When we get into very short barrels (handguns) faster powders are used to get as much speed as possible since the bullet will exit much sooner. The saving grace here is that pressure is allowed to vent before a spike in the pressure curvemay happen because of the very short barrel.

    This is all over simplified for easy understanding but the basic printable is valid.
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    Andy,

    I was using the bore, length of bore and cartridge as a total volume concept based on the third post in this thread. The shorter barrel would result in less volume with all else being equal.

    I don't doubt you I just need to do a little research so I better understand this topic.

    It seems to me that a faster powder would be required in my example since the bullet would be leaving the barrel sooner and pressure building would stop at that point. I do realize that things are not always as they appear in reloading.

    On an other note, my 20" 308 prefers RL-15 over Varget with 175gr bullets and most burn charts show RL-15 to be slower. So that alone spoils my thoughts

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    Quote Originally Posted by marshall View Post
    Andy,

    I was using the bore, length of bore and cartridge as a total volume concept based on the third post in this thread. The shorter barrel would result in less volume with all else being equal.

    I don't doubt you I just need to do a little research so I better understand this topic.

    It seems to me that a faster powder would be required in my example since the bullet would be leaving the barrel sooner and pressure building would stop at that point. I do realize that things are not always as they appear in reloading.

    On an other note, my 20" 308 prefers RL-15 over Varget with 175gr bullets and most burn charts show RL-15 to be slower. So that alone spoils my thoughts
    Ya it's very complex, like how an airplane wing works, there is a heck of a lot going on that’s not evident at first glance. I am just talking in general terms here and not counting things like charge weight, shape of the case (which effects ignition), primer, and a thousand other things that come into play. You are correct that using a faster powder could make up for the loss of length . . . provided the bullet exits before the pressure gets too high, you can find a faster powder that's not too fast, and a host of other factors aren’t somehow changed.

    With powder rates we jump from one speed to the next, to the next, so we may not have a powder that’s in between like some guns would like. Say we are stuck working with a powder that is a tad too fast in a 24” barrel since the next step down is too slow. Lop it to 20” and it may be the perfect burn rate not dropping any velocity at all, I’m sure you have heard of this if not had it happen to you. Now at 20” we may be able to push the charge weight up and get more velocity without pressure signs than we could with that 4” extra on the end due to the pressure spike happening after the bullet is gone.
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    Seems like barrel harmonics needs to be considered also when lopping a barrel off. Adjusting powder and bullets loads is all according to the barrel harmonics of the bullet traveling at a certain speed down the barrel based on the above cartridge information.

    I am too interested in this as I have just bought a Winchester 375 H&H with the barrel cut at 21 inches. Will R22 completely burn before exting the barrel as a big flash or will a faster powder be better?

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    Here is my understanding. Barrel length (unless insanely short) has little to do with the ideal powder from a velocity stand point. It may have an effect from a accuracy stand point as it may cause the bullet to leave the mussel at a time it is not moving at the mussel, however it may be a slower powder in a short barrel that achieves this.

    Powder selection (for velocity) has more to do with case volume as it compares to bore diameter and bullet sectional density. When the primer fires all the powder is burning very shortly after, my understanding is that the time needed to start all the powder burning is near instant relative to the other things happening. Now as the powder burns it puts pressure on the base of the bullet, this pressure is resisted by the inertia of the bullet and barrel friction and the force needed to engrave the rifling into the bullet. The lighter the bullet relative to the bore area (ie lower sectional density) the less pressure it needs to get the bullet to move faster.

    The burn rate of powder is not constant. It depends on temperature. Now gasses cool as they expand. So if the powder is too fast it burns so fast that it can not speed up the bullet fast enough at ok pressures to accelerate the bullet enough to give the gas the room it needs to expand and cool. So the temperature goes up and therefore the burn rate goes up. So we get kind of a self reinforcing increase in burn rate much like a nuclear reactor melting down.

    Conversely if the powder is to slow, the bullet has time to move to far down the barrel before the powder can create enough gas to fill the additional volume and keep the pressure up. So the burn rate slows making things worse.

    So the selection of the correct powder involves finding a powder that will more or less fill the case, and will burn fast enough the keep up with the bullet, with out burning so fast as to take overly high pressures to move the bullet fast enough to create the needed additional volume.

    So looking in a reloading manual you will find for a give bullet weight and diameter, that the larger the case the slower the powder needs to be. So in order of increasing powder speed for a given bullet, we would have 300 RUM, 300 Win Mag, 30-06, 308, 30-30.

    If the bullet has the same inertia (weight), and is fired from a cartridge of the same volume. But, has a larger diameter then it has more area for the gas to work against, so it will accelerate faster at a given pressure and need a faster powder. So in order of increasing powder speed for a given cartridge volume and bullet weight, we would have 260 rem, 7-08, 308, 338 fed, 358 win.

    Now since the peak pressure is reached after a rifle bullet has only traveled less then 8 inches or so down the bore and with rifles we are talking changing lengths out in at 20" or so, changing powders to a faster one will not really help with velocity, if max pressures are kept the same. However, by using less of a faster powder, the pressures can be kept safe and since the powder burns faster there will be much less mussel flash and blast then will the slower powder.

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    I am not so sure of that last statement about barrel length and burn rate as with the bullet at max speed at 8 inches was it?

    In a 35rem H4895 burns faster than TAC does and there is no muzzle flash and I get can get 2200fps. This is with a 18.5" inch barrel. Now TAC in a 20 tube gets 2250fps with the same 200gr bullet, but will only do 2200fps in my 18.5 inch barrel and I get lots of un-burnt powder on my chrony set at 10 feet. And this is say 2.0grs prior to a max load. Adding more powder does not increas velocity but a faster burning powder does.

    So that tells me TAC is to slow for my 18.5 inch barrel and not all the powder gets burnt in that short of barrel but in a 20 inch or longer barrel it is well matched for burn rate with that bullet in the 35rem.

    So far in my 358win with 200gr bullets its H322 over H4895 for velocity with accuracy, and TAC with the 225gr bullets. It has a 20.5 inch barrel #3 Shilen barrel.

    And a 35rem in a 10 barrel will not do what it will in a 16, 18, or 20inch barrel so time under the pressure curve does affect velocity so that bullet unless very poorly matched to a powder is acclerating right to barrel exit or very close to it.

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    Smitty and Andy are correct. Expansion ratio is the culporit and it is the volume of the case plus the bore compared to just the bore volume. Bore volume being greater will use faster powder to keep it moving. However there is also the consideration of the time vs pressure curve, Where will the pressure be when we run out of barrel? Faster propellants are better suited for shorter barrels from a best velocity stand point, all else being equal, bore diameter, bullet weight (travel time) etc, but this is fine tuning and differences between 4831 and 4895 in a short barrel may be only slight, differences being greater for larger bores. this is a good study unfortunately I am due a nap and will have to address the issure in the AM.
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