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Thread: Powder Burn and Pressure Relationship

  1. #21
    Boolit Master 303Guy's Avatar
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    H4895 powder was chosen because it is the slowest burning propellant that ignites uniformly in reduced charges. For years
    H4895 has been the top choice of cast bullet shooters.
    H4895 is faster than Varget.
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  2. #22
    Boolit Master uscra112's Avatar
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    You are right. I should make sure brain is engaged before putting fingers in gear. It's right next to Varget in the Quickload database, but it does come first. As punishment I shall let that post stand.
    So long, and thanks for all the fish.

  3. #23
    Boolit Master
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    What burn there is will occur in the first few inches of bullet travel no matter how long the barrel as long as peak pressures get high enough to ignite the powder. The higher velocities of longer barrels is from their greater expansion ratio not their greater propellant burn.

    Most powders have about the same energy per weight and produce similar amounts of gas. Use of a larger case will allow a greater volume of powder (more gas) to be used at a given peak pressure. For example if one charge burns 20 grains at peak pressure of 30 kpsi and another charge burns 40 grains at the same pressure the latter will produce more gas volume and will be able to maintain the gas pressure longer. The longer the pressure can be maintained the longer the bullet continues to accelerate.

    Barrel pressure does not stay anywhere near constant over any distance of bullet travel much less 30 kpsi to 20 inches our 26 inches.

    A QL check using a 30-30 with a 170 gr GC Lyman RN bullet and 30.3 gr IMR4895 shows 2000 fps @ 20" and 2100 @ 26 ". WOW. A whopping 100 fps difference. The increase in powder burned 20" vs 26" was 3 %. WOW again. Peak pressure 30 kpsi occured at 1.4 inches and decreased to 20kpsi (5 inches), 12 kpsi (10 inches), 9 kpsi (15 inches), 6.2 kpsi (20 inches, 4.65 kpsi (26 inches). If we can not maintain 30,000 psi from the peak at 1.4 inches out to 5 inches how do we expect to maintain it to 20 or 26 inches?

    The advantage of slower powders is not longer burning times. It is the ability to burn greater quantities of powder, producing greater quantities of gases, without exceeding peak pressure limits. Their importance is that they are 'slower' in terms of slower pressure rise rate. Once peak pressure has been reached that advantage disappears. In fact a powder that has a faster burning rate after peak pressure has been reached would yield better performance since that gas would have longer to act on the base of the bullet. Better to have that energy released after 5 inches of bullet travel than after 15 inches.

  4. #24
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    Doc makes several very good points; having measured and observed the time/pressure curves of numerous powders in numerous cartridges the last few years I have to certainly agree with: primarily the expansion ratio of the cartridge has a lot to do with how well a slower powder burns at what psi levels.

    Assuming the "slower burning powders" is refering to rifles I have found the following medium to slow burners to work well (read that ignite and burn reletively evenly) at low psi (down to the 20,000 psi level); 4895 (I prefer H4895), Varget, 4064, RL19, 4350 (I prefer AA4350), RL19, H4831 (I prefer H4831SC) and RL19. The exact bottom end of that level is very dependent on the exapnsion ratio, bullet weight and case volume (load density). The use of a dacron filler when load density is less than 80 - 85% greatly improves the ignition and efficiency of the burn.

    Like Doc I also sometimes have secondary spikes with some cartridges with some powders, even faster burning ones. It's not the gun used or a defect in the gauge mounting as other loads give smooth time pressure curves. I've not yet figured it out......probably time to call Dr. Oehler?

    I'm sure there are others out the but I've not tested all of them so can't say. The above listed powders are generally readily availble, work well in a wide range of cartridges with medium heavy to feavy cast bullets so I've not had a need for a lot of other testing so far.

    Larry Gibson

  5. #25
    Boolit Master 303Guy's Avatar
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    The advantage of slower powders is not longer burning times. It is the ability to burn greater quantities of powder, producing greater quantities of gases, without exceeding peak pressure limits.
    Actually, with rifle powders the burn continues past the peak pressure point of the curve and slow powders burn quite a ways down the barrel. Maybe not all the way down and maybe not even half the way down but further than just the peak. It has been shown that a longer barrel will burn all the powder while a shorter barrel with the same loading will leave unburned kernels (that's with a specific load and powder, not any or all loads and powders). Increasing the powder charge with the specific loading will often burn all the powder. Point is some powders behave differently to others.

    My uncle was an armourer in the dessert in WWII and said that after a 50 Browning machine gun shoot at the range they would sweep up heaps of partially burned powder from in front of the guns.

    Like Doc I also sometimes have secondary spikes with some cartridges with some powders, even faster burning ones.
    That secondary pressure spike has been lab tested and it's not barrel harmonics or some such. They went so far as to pack the barrel with sand bags to dampen vibration and that spike was still there.

    A conclusion by some was that PressureTrace picked up barrel harmonics. In an attempt to prove this theory one shooter even hung a bowling ball off the end of his barrel, but of course there was no change.
    Public debate over whether these secondary spikes are real was finally put to bed when Charley Sisk at Sisk Rifles blew the end off two barrels. We have also verified changes to the rate of acceleration just prior to, and after these events. Case Closed, it is real!
    Last edited by 303Guy; 04-05-2013 at 01:59 PM.
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  6. #26
    Boolit Man
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    Quote Originally Posted by Rocky Raab View Post
    and why such new ones as LilGun and AA#9 can be made to ignite easily but burn slower than they otherwise would if they were monolithic.
    Speaking of - Lil'Gun has my vote. For lack of a better term it shoots "smoother" with beeg boolits. Instead of a "Krrrack!" you get a more rounded "WHOMP". I know my groups have improved with it.

  7. #27
    Boolit Master


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    I never did buy off on the "barrel harmonics" reason for the secondary spikes either.

    Larry Gibson

  8. #28
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    Quote Originally Posted by 303Guy View Post
    Actually, with rifle powders the burn continues past the peak pressure point of the curve and slow powders burn quite a ways down the barrel. Maybe not all the way down and maybe not even half the way down but further than just the peak. It has been shown that a longer barrel will burn all the powder while a shorter barrel with the same loading will leave unburned kernels (that's with a specific load and powder, not any or all loads and powders). Increasing the powder charge with the specific loading will often burn all the powder. Point is some powders behave differently to others.

    My uncle was an armourer in the dessert in WWII and said that after a 50 Browning machine gun shoot at the range they would sweep up heaps of partially burned powder from in front of the guns.



    That secondary pressure spike has been lab tested and it's not barrel harmonics or some such. They went so far as to pack the barrel with sand bags to dampen vibration and that spike was still there.
    This is correct. Powder continues to burn after peak.
    If 200 gr of powder burned in the first inch of a .50 BMG, you have a BOMB.
    It has been the printed word that says all is gone at peak. Attributed to gun writers. I did find that article about expansion ratio. It is false.
    I talked to the powder companies and they just told me "internet idiots."
    Progressive burn escapes the natural world.
    Why not use dynamite? It really goes off all at once.
    Maybe I am stupid but I know all powder is not consumed so fast. Smokeless is not classed as an explosive, it is a flammable. It does take barrel length to burn all of it.
    Then someone said the muzzle flash from a short barrel was from the gas reaching oxygen in front of the barrel. No common sense shown because powder produces it's own oxygen or it would not work. Just maybe it is powder out there?
    Someone is not thinking at all. Compare a rifle with 50,000 PSI using 35 gr of powder with a mag at 50,000 psi with double the powder charge. Did the double charge go off all at once? That would mean 100,000 PSI. Or, Heaven forbid, more distance to burn was needed.
    Expansion ratio just must be more from a higher charge with the same peak pressure---RIGHT???? OH, OH, maybe you are wrong.
    Put it in perspective. Why does a .50 BMG need 200 gr of powder? Just shorten the case to 1" and it will peak the same pressure and depend on expansion ratio.
    Why do the guns on a destroyer need bags and bags of powder behind the projectile?
    A few read gun rags for information, others think.

  9. #29
    Boolit Master


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    Expansion ratio just must be more from a higher charge with the same peak pressure---RIGHT???? OH, OH, maybe you are wrong.

    The "expansion ratio" I was refering to is; "The ratio of the volume of the bore, measured from the base of the seated bullet to the muzzle, to the volume of the cartridge case. An expansion ratio of 9:1 means the bore volume is 9 times the case volume." (Firearms Encyclopedia).

    A .38 SPL revolver with a 6" barrel actually has a greater expansion ratio than many rifles with 20" barrels. There is a maximum expansion ratio at which any given cartridge/load combination will produce maximum velocity. The .243W for example would require nearly 30" of barrel to produce an optimum expansion ratio and velocity. Many of todays "high intensity" cartridges would require barrels of 30 - 40" long to achieve the expansion ratio and efficiency of the 30-30 cartridge.

    Larry Gibson

  10. #30
    Boolit Master 303Guy's Avatar
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    By definition the expansion ratio is the ratio of the volume of the case to the base of the boolit to the total volume. Total volume is volume of case plus bore volume.

    A revolver is going to have a bit of a complicated expansion ratio because one has to take into account the expansion out the cylinder to forcing cone gap.
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  11. #31
    Boolit Master
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    Lets say for argument there is only one caliber in the world and it is a 308 Winchester with a 24” barrel, and there is only one bullet in the world a 175 grain Sierra Match king.

    Now with this one combination we still have all the powders we have, and you made a powder burn rate chart from fastest to slowest at a chamber pressure of 20,000 P.S.I. and you now have a perfect list.

    Now you change your mind and you want a chamber pressure of 30,000 P.S.I., your perfect 20,000 P.S.I. list now has a few changes as to burning rate.

    Now do this again at chamber pressures of 40,000, 50,000, and 60,000 P.S.I. and your perfect list is again changed with each higher level of chamber pressure.

    These changes are with just one gun and bullet but different chamber pressures. Think of the additional changes if we changed just the bullet weight as to the perfect burn rate chart at each pressure level.

    With all the combinations of different calibers, case capacities, expansion ratios, bullet weights and bearing surface, the possibilities are endless.

    Powder burning rate charts are RELATIVE AT BEST! Add to that the lot to lot variables to burning rate of +/- 10% and it can make it seem over whelming.

    The secondary pressure spike is caused by not having enough moment force/ start pressure allowing not fully burning/consumed powder to follow the bullet down the barrel. As we know powder needs pressure to make it burn and with a higher pressure it burns faster. The bullet as it moves down the barrel increases the volume behind it, lowering the pressure of the expanding gases due to a larger area/volume that they now displace. Because the rate of acceleration of the bullet slows some, the not fully consumed powder catches up with the bullet and the powder now has the resistance to accelerate it’s burning speed resulting in the secondary pressure spike. As the caliber goes up the pressure drops faster for every inch the bullet moves down the barrel/bore due to bore volume. A faster powder or a heavier bullet with more bearing surface will help out, and with fillers both reducing the powder capacity within the cartridge case and helping to position it for the primer to ignite it.

  12. #32
    Boolit Master


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    Quote Originally Posted by Doc Highwall View Post
    Lets say for argument there is only one caliber in the world and it is a 308 Winchester with a 24” barrel, and there is only one bullet in the world a 175 grain Sierra Match king.
    .
    I could live with that selection with no issues!!!

    Seriously, good explanation DH!!!

  13. #33
    Boolit Master 303Guy's Avatar
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    What's come out of it so far is the idea to look at largest powder charge for the lowest pressure at some average velocity. That's the powder that will have a slower pressure rise with the longest burn. Look at the same powder at the starting loads to complete the assessment. Then one has to discover how well that powder lights up and burns at lower pressure. BL-C(2) appears to be ideal for cast but others have said it burns dirty at lower loadings. Varget on the other hand burns well at low levels (that's from my own observations). IMR 4007 SSC is a powder that is listed with the lowest pressures and fills the case of the Brit.
    Last edited by 303Guy; 04-05-2013 at 10:07 PM.
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  14. #34
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    303Guy

    The definition I quoted has been around for many years and is the understood defintion of "expansion ratio". Is it wise we rewrite and change definitions? When we do that doesn't confusion generally reign? Or shouldn't we just accept and work with the accepted definition? When we do this isn't there a consistency and continuity to the thought process and discussion over time?

    The slowest powder with the highest load density is an over simplification. The powder still must ignite and burn uniformly for accuracy. Many slow burning pwders won't do that at the low end psi's for many cast bullet loads.

    Larry Gibson

  15. #35
    Boolit Master 303Guy's Avatar
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    Is it wise we rewrite and change definitions?
    No, not re-defining anything - Robert Boyle in 1662 and Jacques Charles in the 1800's did that. Charles was inspired by hot air balloons which were the rage at the time. Remember the gas laws?

    Name:  255fb00a254341fef39d022b9595c13e.png
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    Expansion or compression ratio is defined as;

    Name:  f249c64d236e67dc9575331e1e2878f1.png
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    (Initial volume over final volume).

    Anyway Larry, you defined case volume to bore volume and perhaps that is what is spoken of in gun circles which is fine - I stand corrected (but I call it case to bore volume ratio). However, barrel lengths vary and anyway, when we are considering pressure we are really only interested in boolit sectional density. An example would be the 308 and the 243. Both cartridges would use a similar powder charge range (not too similar) yet have a very different case to bore volume ratio. They do however have a similar boolit sectional density range. So to me, bore volume is a bit of a mute point. It's the expansion rate versus case volume that I'd be interested in. That would take into account velocity (more like acceleration rate) and boolit sectional density. Not all that simple as different burn rate powders are indicated and cartridge efficiencies are different with different bore and a different case to bore volume ratio.

    McGraw-Hill Science & Technology Dictionary:
    expansion ratio

    Top
    Home > Library > Science > Sci-Tech Dictionary
    (ik′spanˇshən ′rāˇshō)
    (fluid mechanics) For the calculation of the mass flow of a gas out of a nozzle or other expanding duct, the ratio of the nozzle exit section area to the nozzle throat area, or the ratio of final to initial volume.
    (mechanical engineering) In a reciprocating piston engine, the ratio of cylinder volume with piston at bottom dead center to cylinder volume with piston at top dead center.


    Read more: http://www.answers.com/topic/expansi...#ixzz2PfVE6LsQ
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  16. #36
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    Quote Originally Posted by Larry Gibson View Post
    Expansion ratio just must be more from a higher charge with the same peak pressure---RIGHT???? OH, OH, maybe you are wrong.

    The "expansion ratio" I was refering to is; "The ratio of the volume of the bore, measured from the base of the seated bullet to the muzzle, to the volume of the cartridge case. An expansion ratio of 9:1 means the bore volume is 9 times the case volume." (Firearms Encyclopedia).

    A .38 SPL revolver with a 6" barrel actually has a greater expansion ratio than many rifles with 20" barrels. There is a maximum expansion ratio at which any given cartridge/load combination will produce maximum velocity. The .243W for example would require nearly 30" of barrel to produce an optimum expansion ratio and velocity. Many of todays "high intensity" cartridges would require barrels of 30 - 40" long to achieve the expansion ratio and efficiency of the 30-30 cartridge.

    Larry Gibson
    That is correct Larry. It is something that changes.
    What I referred to was using the term to say more powder going off with a slower powder still going off in the same distance as a fast powder will add more expansion ratio without increasing pressure. It has been used to show a lot of powder just adds more gas at the start and only that gas pushes the boolit.
    We know that can't be done without destroying the gun.
    It is the definition of an explosion.
    If it were not true, we could just double the charge of Bullseye to double the gas and so called "expansion ratio." Imagine a .50 BMG case full of Bullseye!
    It started as a gun writers pipe dream and still continues today with the thought that all powder is consumed in an inch or so.
    It is why some think all powder is burned in a 2" .500 S&W, you really need to fill that huge case with Bullseye. Gun parts really reach velocity that way. (DANGER)
    Larry, I don't now where that thinking came from but it is still repeated.

  17. #37
    Boolit Master
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    By definition the expansion ratio is the ratio of the volume of the case to the base of the boolit to the total volume. Total volume is volume of case plus bore volume.

    303Guy, this is my understanding of expansion ratio also.

  18. #38
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    303guy, Doc

    The expansion ratio of gases as expressed by Boyle is correct as it relates to the total volume the gas has to expand into. However, the "expansion ratio" in reference to firearms expresses the differences in case capacities to bore capacities (volumes). That is the difference; the result means two different things. My quote was not my definition but the standard definition taken from the Firearms Encyclopedia book on definitions of firearm terms for "expansion ratio" as it relates to firearms, not to gases in general. It is also used in other books/articles discussing the subject for many years. I believe it also is the definition used by Powley for his computations as he describes "expansion ratio as "total volume inside the case plus barrel exposed to gass, divided by the case capacity.." Thus again we see as related to firearms/cartridges we have a ratio between case capacity and the total volume for the gas to expand into. A bit different from the simple total volume are of expansion as expressed in Boyles gas laws.

    That being as it may we are essentially saying the same thing. Earlier I posted "The exact bottom end of that level is very dependent on the exapnsion ratio, bullet weight and case volume (load density).". Your use of "sectional density" equates to "bullet weight" as bullets of equal SD in .30 and .243 caliber are comparatively the same in relation to the expansion ration of the .308W vs the .243. Same thing just different terms.....still the same page of the hymm book.....as it relates to your original question.

    Powley also states that for a good working pressure the only 2 factors involved are the expansion ratio and the powder charge to bullet weight. That is what we are basically discussing except that at the lower psi's of cast bullet loads the psi level at which the powder itself will ignite and burn efficiently also must be considered. That is important because regardless of "burn rate" every powder has it's own psi level based on composition and retardent used for efficient ignition and burn.

    Larry Gibson
    Last edited by Larry Gibson; 04-06-2013 at 10:18 AM.

  19. #39
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    Quote Originally Posted by 303Guy View Post
    Varget on the other hand burns well at low levels (that's from my own observations).
    Yes, my observations parallels yours in that Varget burns well and it does better than IMR 4064 for me; Varget is also more consistent throughout the year as ambient temperatures change. I thought about trying H4895 but chose to stay with Varget to have a bit slower powder to fill the case more.

  20. #40
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    44man

    Another quote from Powley if I may which adds to your premiss; "a gun with low Expansion Ratio and a high ratio of Powder Charge to Bullet Weight will be affected much more by barrel shortening than will a gun of both high Expansion Ratio and low atio of Powder Charge to Bullet Weight.

    Larry Gibson
    Last edited by Larry Gibson; 04-06-2013 at 10:21 AM.

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