velocity

Discussion in 'Reloading' started by trueblue, Aug 18, 2008.

  1. trueblue

    trueblue Well-Known Member

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    I have a question concerning velocity and the amount of powder it takes .
    How is it that a 300RUM shooting a 200 gr. bullet max out at approximately 3000fps, but a 338 EDGE based on the same case can shoot a 300 gr bullet at 2850fps. They both share the same parent case, same case capacity, but the 338 shoots a bullet 50 % heavier and only gives up 150fps in velocity. I assume it has something to do with working pressure and the time duration of the bullet in the barrel,but still don't understand how this works, if they are operating at the same pressures. Granted the EDGE velocity is based on a 30" barrel versus a 26" barrel on the 300RUM, but I am curious as to the corelation of powder charge and derived velocity. Would someone please explain this.
     
  2. AJ Peacock

    AJ Peacock Well-Known Member

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    I'm not qualified to completely explain it. However, I believe I know some of the factors that contribute to this phenomenon.

    1) The larger diameter bullet has a greater surface area exposed to the pressure of the powder. Even though the .338 is only about 10% larger than the .30 caliber bullet, the surface area is around 27% more.

    2) As you mentioned, most of the Edge's have longer barrels. Not only does this give the bullet longer to gain speed under pressure, it also makes a more efficient use of the powder charge (ie. more of the powder is burnt while the bullet is in the barrel).

    3) Heavier bullets enable the use of slower powders, compounding the first 2 reasons above.

    Compare the .308 winchester and the 338 federal. Same cartridge, except for the diameter of the projectile.

    I'm sure that someone will point out any errors I've made in the above (I wouldn't be surprised if I've made at least one :confused: )

    AJ
     

  3. davewilson

    davewilson Well-Known Member

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    the surface area being pushed against makes up about 90% of the difference in velocity. a slower powder might not always be the case with a heavier bullet. a 338 has a much higher expansion ratio and might be able to use the same powder.
     
  4. Varminator 911

    Varminator 911 Well-Known Member

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    I'd say your velocity numbers are a bit off. Nosler #6 shows ~ 3200 and 3000 for the 200 and 220 in 300RUM. They show right at 3200 for a 225 in 338RUM or 338/378 WTBY all in 26 inch barrels. That's not that much difference. So I'm thinking 3000 is way slow for a 200 in 300RUM and 2850 is generous in the Edge with a 300.
     
  5. goodgrouper

    goodgrouper Well-Known Member

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    I can get 2850 with a 300 in my "edge like" wildcat pretty easily and I have gotten 3300 fps with 200 grain bullets in many 300 ultras but they usually shoot better between 3160 and 3225 fps. These are all 26" barrels by the way.

    It all has to do with expantion ratios and pressure curves of given powders like Dave Wilson said. Internal ballistics is a pretty detailed and involved science but the good news is that all that has already been figured out by the powder producers and all you have to do is look at a burn rate chart and start from there.
     
  6. jwp475

    jwp475 Well-Known Member

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    I was a little confused with True Blue claiming a 300 RUM maxed out at 3000 FPS with a 200 grain bullet. I have only worked with mine but I ran 200 grainers 3250 FPS with zero problems
     
  7. trueblue

    trueblue Well-Known Member

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    I don't have a 300RUM, so the 3000fps was just an example . The examples of speed given can be questioned, but that does not change the point of the question.
     
  8. jwp475

    jwp475 Well-Known Member

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    The answer is the expansion ratio. The same as a hydrolis cylinder the larger the diameter the more it can moved at the same presure.
    I believe that Dave already answered your question.
     
  9. linksmechanic

    linksmechanic Well-Known Member

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    Actually it does as your listed speeds are inaccurate. The rum will run a 200 grain around 3200 and a .338 edge 2850 so the rum really doesn't give anyting up to the edge in speed only the ability to shoot a massive amount of lead at a descent speed.
     
  10. trueblue

    trueblue Well-Known Member

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    Thanks for your replies.
     
  11. AJ Peacock

    AJ Peacock Well-Known Member

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    Links,

    What load are you getting that kind of velocity with? According to Quickload, even 3100fps is over pressure with nearly every powder (this with a 28" barrel).

    AJ
     
  12. jwp475

    jwp475 Well-Known Member

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    I don't know about quick load but I easily got 3250 with the 200 AcuuBond and Retumbo and gg said in an ealier post that he has gotten 3300 in several RUMs
     
    Last edited: Aug 19, 2008
  13. linksmechanic

    linksmechanic Well-Known Member

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    I got 3190 with a 210 berger with no pressure using retumbo. This was last year so I will try to dig up my data.
     
  14. AJ Peacock

    AJ Peacock Well-Known Member

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    Those velocities are very impressive. According to Quickload, to get those velocities in a 26" barrel, the pressure would be just over 88k PSI with Retumbo. The spec is for a max pressure of just under 64k PSI.

    Edited: I looked closely at the settings that Quickload has for the Rum's and it appears the weighting factor for those cartridges was set for more of a cylindrical case instead of 'overbore'. So I am guessing that the pressure is less than Quickload is computing. Just FYI

    Drop me your loads and I'll play with the settings to match your chronograph readings. Thanks AJ

    I sure hope Quickload has something incorrect. What is your OAL for that round? As it does make a difference in that the longer you seat the bullet, the lower the pressure (until you get against the lands).

    AJ
     
    Last edited: Aug 19, 2008