Temp sensitivity

Discussion in 'Reloading' started by Thumper1991, Apr 19, 2013.

  1. Thumper1991

    Thumper1991 Active Member

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    My question is how do you find the temp sensitivity of your gun powder. I am guessing Ave on a chrony. I would think though this info would be published but I can't seem to find it. Right now the powder I am working on is h4831sc. Thanks for the help
     
  2. varmintH8R

    varmintH8R Well-Known Member

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    You can do it on a chrony, or by looking at poi changes and backing that into velocity. H4831sc is an extreme powder and should be relatively stable.
     

  3. azsugarbear

    azsugarbear Well-Known Member

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    Even with temp sensitive powder, you will have some big changes in POI with 20 degree change in temp. Every little thing counts in extreme LR shooting. If you are serious about finding out, get an infrared thermometer from Harbor Freight for $21. Put your cartridges in the freezer the night before. Transport them to the range in a cooler, then pull them out and shoot them over a chrony every 20 degrees of increase - say 30, 50, 70 and 90 degrees (for us AZ rats). You may be surprised at what you find.
     
  4. Bart B

    Bart B Well-Known Member

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    How does one measure the temperature of powder inside the case with an IR thermometer?

    Pointing mine at a cartridge only measures the temperature of the brass case. Cartridge brass does not have the same temperature rise coefficient as smokless powder. The heat transfer coefficient of brass is 315; copper jacket's 389. For cellulose (closest thing to smokeless powder I could find) is .039. So the case will heat up much faster than the powder.

    I have observed that the longer a cartridge stays in a hot chamber, the faster the bullets leave. With a .308 Win. cartrodge, every 30 seconds of chamber time requires a 1/4 MOA reduction of sight elevation for 1000 yard targets. 30 caliber magnums need a 1/4 MOA change down after about 20 seconds.
     
  5. Mikecr

    Mikecr Well-Known Member

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    There is no published data to even try & apply for this. Also, you should not allow marketing hype to lead into assumptions.

    It's just an area that has to be tested in your gun, with your lot# of powder & load, at different conditions. If you plan to use the same barrel for a wide spread of conditions, then you can keep a log and expect compromise across some of the range.
    I have summer guns that never leave the safe until temps get above 85degs. They're used only for hunting at high temps, where my hunting loads were developed, so I don't burn up any barrels shooting at lower temps -in an effort to produce some sort of universal guns.
    I also keep ammo in my pants pockets, while load developing, and while hunting, single loading rounds just prior to actual shots. I do this hot or cold for any hunting gun. I also keep a hunting barrel acclimated to the field temps, by keeping them in shaded cases outside during hunts. They can take a couple hours to stabilize otherwise.
     
  6. TOM H

    TOM H Well-Known Member

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    I read some of your prior post seem like your pretty good hunter on deer/elk plus loading for a friend and he did pretty good with your loads using H-4831sc.

    Since you live in Idaho maybe you could find place close to where you hunt check you POI at varies yardage. I use H-4831sc in my 30-338mag and I've had that rifle out to maybe 20 below on elk here Co never had a problem with it. I know Hodgdon put something out on their powder not sure it comes into play in the real world of hunting.

    Well good luck
     
  7. FEENIX

    FEENIX Well-Known Member

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  8. Greyfox

    Greyfox Well-Known Member

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    I have found the temperature stable powders to work quite well. I have used Retumbo for the last three seasons from 25-75 degrees with little or no change in impact points out to 1000 yards. I hunt Wyoming in October, and Alberta in November. I have chronographed velocities over this same temperature difference and found average velocities to be within 10-20FPS. I don't trust my chronograph as much as I do actual drop testing at long range though.
     
  9. 1100 Remington Man

    1100 Remington Man Well-Known Member

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    I could not get what I wanted in my .264 Win Mag with H-4831. Has anyone tested Alliant powders for temp sensitivity ? Or IMR powder ? I will have to try Retumbo thanks Greyfox.
     
  10. Bart B

    Bart B Well-Known Member

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    Those powder company folks probably have. Contact them then ask about it. If they have, their data's the best on actual issues.

    Has anybody "testing" ammo by shooting it actually measured the temperature of the powder in the cartridge when it shoots the bullet out? I doubt it's the same as the ambient air temperature; most likely warmer in cool/cold weather. And one must shoot the chambered round within 1 second of closing the bolt on it so a warm/hot barrel doesn't heat up the case and the powder while it's in the chamber.

    I doubt a 25 to 75 degree F change in powder temperature would shoot bullets close enough to the same muzzle velocity to have the same drop at 1000 yards with the same ambient temperature for both the bullet goes through. Note cold air has more density than hot air, so a bullet leaving at the same speed in each will drop more in the cold air than the hot air. And the 25 to 75 degree F powder temperature used in the same air temperature will end up with more drop at 1000 yards with the colder powder. If air and powder temperature were the same, the cold powder in cold air would cause even more bullet drop; less energy plus more dense air equals more bullet drop.
     
  11. azsugarbear

    azsugarbear Well-Known Member

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    Testing the temp of powder is probably a misnomer, as Bart B pointed out. But I suspect the relationship between the temp of a brass casing and that of the powder move together in the same direction with some strong correlation.

    As long as you are consistent in measuring the temp of the brass, you have a standard against which you can record changes in velocity. Hodgden Extreme powders seem to do much better than others, but velocities still change. Here in AZ, I have found that as you move closer to the 90 -100 degree mark, the more pronounced that change becomes. I have a load for my 6.5-06 that does quite well at 50-70 degrees. But at 90 degrees, I get substantial pressure signs.

    Being aware of your ammo temp and knowing how it behaves in different temps and conditions - allows you to compensate.
     
  12. Bart B

    Bart B Well-Known Member

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    How does one measure the temperature of the brass while it's in the chamber?
     
  13. FEENIX

    FEENIX Well-Known Member

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  14. azsugarbear

    azsugarbear Well-Known Member

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    Measuring temp of brass/powder depends largely on the situation. As mentioned previously, some keep ammo on their person until it is time to shoot, while others keep it with their rifle and assume ambient temperature. Others don't make assumptions and use infrared right before putting the round into the chamber. Either way, they don't put it into the chamber until right before they shoot. Again, this probably doesn't matter much at shorter ranges, but when start stretching it out beyond 800 - 1000 yards, you ignore it at your own peril. Where I hunt, I often see swings of 50 degrees within five hours, more if in direct sunlight.