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Thread: Slow Powder, Fast Powder,...what's it all about again ?

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    Member kodiakrain's Avatar
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    Default Slow Powder, Fast Powder,...what's it all about again ?

    So, loading up as many spare boxes of .277 bullets as I can these days

    I have some Sierra 130 gr Spitzers, that just Did Not Work with IMR 4350, at at the range yesterday,...
    was surprised at the poor grouping all the way up the scale

    So, trying to think experimentally, I have a pound of IMR 7828 ssc
    that I bought and haven't used much
    Don't have my Propellant Profiles book around, lent that out,.....darn

    Thought I'd ask here, as it isn't listed as an option, for the 130 Sierra,
    but it can be found for the heavier bullets, like the 160gr Partition,..

    "What is the reasoning behind going with a "Slower," powder like that,....??
    and should I spend any time trying to make that work for as light a bullet as the 130gr Spitzer ??

    Maybe more interesting, "If you don't have, Book Recommended, Min to Max, numbers,..
    for a specific powder, can you just wing it, carefully of course,...and try something out like that ??

    Any ideas for me,...
    and yeah, I have plenty of RL-17, (my miracle juice for the wsm),...H4831sc, (Most Accurate for everything, so far)
    and RL-15 to use,...but am looking to use up this 7828ssc on something

    Thanks, for all the experience out there
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    Supporting Member iofthetaiga's Avatar
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    While not as good as the book, lots of propellent profile info can be dug up online.

    This is my favorite burn rate chart: http://www.google.com/url?sa=t&rct=j...41524429,d.cGE
    I find it much more useful for making comparisons than the linear lists.
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    Member kodiakrain's Avatar
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    Thanks, for the chart,...that's a good one,..

    To rephrase my wondering,...I have never seen an explanation for why slower powder burn rate is better for larger bullets
    and the other way around

    On this chart, for the powders I use predominately,...for this caliber .270wsm,...
    there's really not a big spread, from those that work extremely well RL-17 for example
    and IMR 7828ssc

    and RL-15 is only listed for the smallest bullets I can find

    Why is that,..is there a Basic Explanation,...am I wasting my time trying to stretch that understanding a bit ??
    Ten Hours in that little raft off the AK peninsula, blowin' NW 60, in November.... "the Power of Life and Death is in the Tongue," and Yes, God is Good !

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    Supporting Member iofthetaiga's Avatar
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    Way too many variables involved with internal ballistics to provide a quick, simple answer. And I will be the first to admit that I don't know everything there is to know about it (no one does, yet), so I'm not going to even attempt an explanation here. Fortunately, smarter minds have already published lots of good info online about it. A good primer here on the relationship between case capacity/powder charge/burn rate/pressure curve...: http://www.frfrogspad.com/intballi.htm Plenty more available via google "internal ballistics". Best luck. Be safe.
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    good question there Kodiak....I admittedly know very little bout the subject but I am very interested and have another question on similar lines but will start another thread so as not to hijack yours///

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    Quote Originally Posted by kodiakrain View Post
    Thanks, for the chart,...that's a good one,..

    To rephrase my wondering,...I have never seen an explanation for why slower powder burn rate is better for larger bullets
    and the other way around

    On this chart, for the powders I use predominately,...for this caliber .270wsm,...
    there's really not a big spread, from those that work extremely well RL-17 for example
    and IMR 7828ssc

    and RL-15 is only listed for the smallest bullets I can find

    Why is that,..is there a Basic Explanation,...am I wasting my time trying to stretch that understanding a bit ??
    You need to tailor the powder to the cartridge/bullet. An example would be RL-15 for the .500 A-Square. This launches a 600 grain bullet at 3400fps, and the case is virtually straight with no shoulder. This is a high-expansion rate powder. It starts relatively slow but burns faster later in the curve.
    You'll need a slow powder for say the .264 Win driving a heavy 140-160 grain bullet, you'll need a very slow powder as the case is quite overbore. This allows more time in the initial phase of launch to get the bullet moving without too high chamber pressure.
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    Ok, I'll take a stab at this. Basically the rule is faster powders for lighter bullets and slower for heavier bullets. You are a boat guy so this might be easy. Take your boat sitting at a dock and run as fast as you can and slam into the side of it. It will move a little. Now stand beside it and push slowly on the boat. It will move probably more than the fast hit. Now take a canoe and do the same. The lighter canoe will make better use of the fast hit and the heavy boat will make better use of the slow push. Bullets and powder are very similar. A light bullet has less mass so it moves easier producing less pressure. A heavy bullet has more resistance to movement so you don't want to run at it so fast. Slower longer burn to keep the pressure from getting out of hand while the bullet is starting to move. Now there are a lot of other factors involved as powders have been doctored to control their burn rate and don't usually burn at a linear rate. So even though two powders may be very close on a burn rate chart, it doesn't mean they will react the same way in any given cartridge or gun. It's part of why we do so much searching for the perfect load for our guns. I hope that didn't muddy the waters too much.

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    First check out hodgdons web page they have load data there for a great many cartridges including 130 grain 270 WSM and IMR 7828. They are listing Hornaday bullets not Sierra's. Personally I see no issue with swaping bullet make with load data provided they are of similar construction as is the case here.

    To answer your slow verses fast question lets look at how a bullet moves. The pressure acts on the back of the bullet creating a force Equal to pressure times area. The area of the back of your bullet can be approximated by finding the area of a circle with a diameter of the average of bore and grove diameters in your case Area = D*D/4 * Pi = 0.2735" * 0.2735" /4 * 3.1428 = 0.0587 in^2. Off the top of my head the 270 WSM has a Max average pressure of 65,000 psi. So the max force on the back of the bullet is Force = pressure * area = 65,000 lbs/in^2 * 0.0587 in^2 = 3820 lb. this max force will be the same for all bullet weights and types. Now what does this force do? Some goes into engraving the rifling into the bullet, some goes into friction with the bore, but the majority goes into acceleration the bullet. For now lets assume all the force is used to accelerate the bullet.

    Okay now going back to physics. Force = mass * acceleration or F =m * a. If we resolve for acceleration we find that a= F/m. notice that for a given force the heavier bullet (ie more mass) will lead to a lower acceleration, while a lighter bullet (ie less mass) will lead to a larger acceleration.

    While not needed for purposes of this discussion it is interesting to look at the values of acceleration we get assuming all the force on the base of the bullet goes into accelerating it (ie no friction or engraving force). Note: weight of bullet in pounds = weight in grains / 7000 = 130 /7000 = 0.0186 lbs. We can find the value of acceleration in gravities (g's) by using the following formula: a (g's) = Force / weigth of bullet (pounds) = 3820 lbs / 0.0186 lbs = 205,700 g's of acceleration. That is right under peak chamber pressure the bullet is being accelerated at a rate of over 200,000 gravities! Of coarse it really is a little less than that due to the friction and engraving forces, but not much less. And of coarse heavier bullets would have less acceleration while light bullets would have more.

    Okay back on topic: lets take a very generalized look at what happens when the powder burns. Two products are created Gas and Heat. The gas takes up much more volume then the unburned powder so the pressure behind the bullet increases as the gas is forced to compress. Compressing a gas produces heat this is why the outlet from a air compressor gets so hot. Conversely reducing the pressure of a gas sucks in heat this is why air tools get cold or why the valve stem on a tire gets cold as you let the air out. Now as the bullet accelerates down the barrel more volume is available for the gas so, the pressure lowers and the gas cools further lowering temperature. Of coarse as the bullet accelerates opening up more volume for the gas the powder is continuing to burn producing more gas and heat, which may be more or less then the volume and cooling provided by the additional volume.

    Now the rate at which a the powder burns is directly related to the temperature. So the hotter the temperature the faster the powder burns. The lower the temperature the slower the powder burns. When the primer fires the powder is ignighted and starts to burn in from the outside of the grains. This builds pressure and heat which speeds the rate of powder burn. This creates a potentially dangerous situation, if the powder burns faster than the volume of gas and heat can be released by the acceleration of the bullet then the temperature rises and the rate of burn increases which leads to a runaway increase in pressure and temperature which can blowup the gun. On the other hand if the powder burns to slow, then the bullet opens up new volume to quickly and the temperature drops leading to a runaway slow down of the burning and low velocity and unburned powder in the barrel are the result. So we need to walk a reasonably fine line with burn rates we want fast enough but not to fast. Since the rate of acceleration of the bullet at a given pressure is inversely proportional to its weight (ie heavy bullets accelerate slower) we need slower powders for heavy bullets and faster powders for lighter bullets when dealing with a given case capacity, allowable pressure, and bullet diameter. Also bullets with higher engraving or friction forces may need less powder or slower powder to stay safe, as these forces will lower the acceleration of the bullet.

    When the primer fires all the powder starts to burn instantaneously (not exactly true but from what I have read this is assumed to be close to true). When the pressure gets high enough the bullet starts to accelerate and the volume increases by the amount of new volume behind the bullet. Keep in mind given the same pressure, bullet diameter, bullet weight, engraving, and friction forces the acceleration is the same no mater how big the cartridge case. So, for a relatively small case (say 308 win) the volume increase from the bullet moving say 1" is the same as for a large case (say 300 RUM for example). However, the small case is burning a much smaller mass of powder than the large case, so the small case needs to burn its small amount of powder quicker to keep up the pressure, and temperature and therefore burn rate. So small cases need faster powders for a given pressure, bullet diameter, bullet weight, engraving, and friction forces; while large cases need slower powders.

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    Hey now,...that's pretty impressive, rbuck
    I can totally get that picture you painted there,..Thanks,
    from a boat guy perspective,...you're right, makes total sense

    So that being considered, with a 130gr bullet,...kind of midrange as .277 options go (from 90gr to 160gr)
    and IMR 7828ssc,...a notably slower powder in those suggested for that caliber in wsm

    Do you think I can get away with it,...will it be worth the effort to see if i can make a work-up with that powder ??

    I ran into some stuff searching all around the links I've been given and did find a few comments
    on 7828 producing some good results for 140 gr, so I'm going to try it
    Ten Hours in that little raft off the AK peninsula, blowin' NW 60, in November.... "the Power of Life and Death is in the Tongue," and Yes, God is Good !

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    Looking at the link you posted, iofthetaiga, it looks like IMR 4350 and RL19 are almost identical in burn rates. Could I use min starting data for IMR4350, but loading RL19, in a wildcat 35-338WM? I had another chart that showed 5 powders between IMR4350 (faster) and RL19 (middle), but 3 powders between RL-19 and IMR4831(slower). I am looking at 250 gr Hornady Sp. I have RL19, not enough of the IMR's to get serious about load development.
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    Also consider that powders regardless of burn rates require the proper length barrel to burn and produce the desired pressures/velocities. A super short barrel that's maybe 16" long will produce a massive muzzle flash/noise with a slow burning powder, because a significant volume of powder is being burned after it is blown from the barrel. The same kind and volume of powder fired in a 24" barrel might have no muzzle flash and a much higher velocity. Also the proper propellant pressure curve is required for properly cycling semi-auto rifles. If the powder burns too fast the barrel has too much pressure when the bullet passes the gas valve and the action gets battered, if the pressure curve is too slow the action might not cycle at all as the pressure hasn't peaked before the bullet leaves the barrel. Everything is a trade-off, longer barrels make more velocity with the right powders but do you really want to carry a rifle with a 28" barrel.

    A good test for most rifle powders is to shoot them late in the evening and see if the muzzle flash is abnormally large this might suggest the need for a different powder. Back when muzzle loaders ruled the west, the old-timers would shoot thier rifles in the evening over snow and observe the flash, and check the snow for unburnt powder; using this info to adjust their loading.
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    Supporting Member iofthetaiga's Avatar
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    Quote Originally Posted by calm seas View Post
    Looking at the link you posted, iofthetaiga, it looks like IMR 4350 and RL19 are almost identical in burn rates. Could I use min starting data for IMR4350, but loading RL19, in a wildcat 35-338WM?
    Personally, I like that chart because it displays the powders relative to one another such that it helps me extrapolate more effectively than the linear listing does... That being said, there's more to the characteristics which differentiate one powder from another than "speed" (just because two powders look to be about the same speed doesn't mean they're both appropriate in a given cartridge), and I don't know enough about those particular powders or that cartridge to offer any advice. Somebody like Murphy might be able to weigh in.
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    No expert here but I think I have a basic grasp. In addition to heavy/light bullets and shorter/longer barrels, considering how overbore the cartridge is also a factor. I shoot a lot of cartridges that tend to be on the overbore side and I like to go with the heavier high BC bullets. So I use powders in the RL17 to Retumbo range for the most part. And just recently ordered some of the new RL33, which is very slow, for 230 gr bullets in the 300 RUM as well as 100 gr + bullets in a 6mmx284.

    7828 ssc should work well in a 270 WSM with heavier bullets as it is an overbore cartridge about like a 7 RM. I just recently worked up loads for my 7 RM with RL17 and Retumbo and 160 AB's. Both did well but the RL17 gave me 50 fps more velocity... but it only filled about 85% of the case and I like a minimum of 90%. The Retumbo filled to about 98%.
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    the volume to weight ratio does not necessarily correlate to the burn rate.

    this means a burn rate of one may occupy very little space in a case while the other may use all of the available space for a given weight. the first will be very sensitive to changes or mistakes. so, no, do not just compare burn rates and assume you can substitute one for the other.

    there are ,however, some powders that have granules that are mostly the same [ in size and shape and weight] and burn rates that are close and folks can and do substitute them with good results.

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    Quote Originally Posted by Nitroman View Post
    You need to tailor the powder to the cartridge/bullet. An example would be RL-15 for the .500 A-Square. This launches a 600 grain bullet at 3400fps, and the case is virtually straight with no shoulder. This is a high-expansion rate powder. It starts relatively slow but burns faster later in the curve.
    You'll need a slow powder for say the .264 Win driving a heavy 140-160 grain bullet, you'll need a very slow powder as the case is quite overbore. This allows more time in the initial phase of launch to get the bullet moving without too high chamber pressure.
    600 at 3400? You sure about that?

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    I have a chart similar to the one iofthetaiga lists, that I have on the wall in my shop. I use a highlighter on the powders that I have, so it is easy to see if I have a certain powder or one close to the same burn rate. Lately I've had to replace it due to the fact I have run out of some of the powders and can't replace them.

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    Quote Originally Posted by Secondhand Bob View Post
    I have a chart similar to the one iofthetaiga lists, that I have on the wall in my shop. I use a highlighter on the powders that I have, so it is easy to see if I have a certain powder or one close to the same burn rate. Lately I've had to replace it due to the fact I have run out of some of the powders and can't replace them.
    Is this custom-made, or ordered from somewhere, Bob?
    Don't want no one to get hurt, but if yore gonna have a wreck, I wanna watch.

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