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powder charge versus velocity, always linear?

Discussion in 'Reloading' started by Andrew Tacquard, Jun 10, 2014.

  1. Andrew Tacquard

    Andrew Tacquard

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    Tried posting this yesterday, guess it was deleted.

    I noticed that my powder charge versus velocity plots were somewhat linear with my bullseye loads. I was loading to make major, and worked up a load to about 168 PF. So when I was making my subsonic 300 BLK loads I made three loads (that I knew were well within the safe region), plotted them, fit a line, plugged in the desired velocity to my equation and it spit out a charge. I loaded to the charge and I was within a few fps (equation was correct). I am trying the same thing with WST for my new pistol loads, again loading major. I made two loads this time (and assumed a linear relationship), got the data, made an equation and loaded rounds based on that. I have not chronoed those loads yet, but I suspect they'll be darn close.

    Is powder charge versus velocity mostly linear when your staying away from max charges? I know most everything in nature is non-linear and I suspect at the low end and high end it will be non-linear as well. However, if this holds up it sure makes my work up process faster and easier when I loading to a velocity.
     
  2. WeeWilly

    WeeWilly

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    In my experience it is not always linear. I think the conclusion that it is somewhat linear is more a function of how small a part of the pressure curve you are inspecting/measuring, than anything hard and fast.
     

  3. Colorado4Wheel

    Colorado4Wheel

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    In the middle part I have found it to be somewhat linear.
     
  4. fredj338

    fredj338

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    My exp, it depends. For most powders, as C4W notes, vel will be linear in the midrange. With faster powders, the pressure curve steepens as you get closer to max, vel can also go up OR they can flatten. If you are adding powder & getting no appreciable increase in vel, it could mean you have reach working max for that powder. Trying to make major for what caliber? IMO, not really safe with fast & uberfast powders, you run the risk of a pressure spike as you near the top end.
     
  5. Andrew Tacquard

    Andrew Tacquard

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    10mm.
     
  6. jmorris

    jmorris

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    It will never be linear. At some point above 0.0 grains the bullet will leave the barrel and at some point over max it will remain in the barrel as the rest of the gun explodes.

    A good rule of thumb reloading is to never speculate. Test, test and test, then record you data (and post it).
     
    Last edited: Jun 10, 2014
  7. mboylan

    mboylan

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    Fast powders are often very non-linear close to and over the max.
     
  8. fredj338

    fredj338

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    Few powders faster than w231 are safe for 10mm max performance loads. Lets call that 180gr bullets @ 1200fps. High pressure rds run that way, need powders slower than w231.
     
    Last edited: Jun 11, 2014
  9. WeeWilly

    WeeWilly

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    So my guess in the pool is your WST loads will end up +.2gr over your Bullseye major load, everything else being equal (including OAT),
     
  10. Colorado4Wheel

    Colorado4Wheel

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    Since we are talking 10mm. The stock G20 will start opening too fast around 1150fps with a 180gr bullet. So you get erratic ES/SD as a result. A heavier spring will solve this problem. To me your driving the gun past what it is designed to do at that point. But most people think the 10mm is actually a MAGNUM so they just throw a heavier spring in and up the charge but another 10%. A heavier spring will setttle the ES down and let your shoot loads that are easier to understand over the chrono because they are not as erratic.
     
  11. WeeWilly

    WeeWilly

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    I must have missed something, which certainly will not be the first time, but by my estimate you need about 920fps with a 180gr bullet to make major. Call it 950fps for a margin for error. That should be pretty easy with WST and 10mm.
     
  12. fredj338

    fredj338

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    Or any of the powders available. The definition of major needs to be qualified though.:dunno:
     
  13. WeeWilly

    WeeWilly

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    I was assuming the OP's meaning of Major was 165pf, perhaps that is where I missed the boat.
     
  14. Andrew Tacquard

    Andrew Tacquard

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    WeeWilly your the only one on the boat with me...

    Sorry I assumed everyone knew what major PF meant, I figured by including my goal of 168 that would narrow it down. That explains all the concerns for over pressure, running to fast, fast powder, etc. So from the current USPSA rule book "Power factor is calculated using the bullet weight and the average velocity of the three rounds fired, according to the following formula:
    Power Factor = bullet weight (grains) x average velocity (feet per second) / 1000". I am running 200 grain lead (and next is coated bayou bullets), I need 825 fps to make "Major"; which from the USPSA rule book is 165 (using the above formula). I settled on 168 as it gives me some margin for weather changes or slow chronos. 168 requires a 200 grain bullet to be moving 840 fps.
     
  15. Andrew Tacquard

    Andrew Tacquard

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    I am not speculating, I am collecting data via a quasi scientific method, analyzing it, fitting a curve (in this case a line), and drawing conclusions from my results. Then I verify my results with further testing, pretty much the standard process for developing anything. Since I have been looking at the data, it has been dang near linear in the area I've been testing. R^2 values of .95 and .97. I realize at the ends it is non-linear; however I am no where near 0.0 or max loads.
     
  16. Andrew Tacquard

    Andrew Tacquard

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  17. Andrew Tacquard

    Andrew Tacquard

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    almost, 4.6 BE and 4.7 of WST, still need to chrono to verify.
     
  18. WeeWilly

    WeeWilly

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    Sounds like it should be very close.

    Fred was the first person I read to point out the reverse temp sensitivity of WST. I shoot where the temps are very consistent and mild, so I don't have to worry about it, but as the temps go up as the summer wears on, your numbers may change. Maybe build in a little additional margin, you probably have some headroom if needed.
     
    Last edited: Jun 11, 2014
  19. Andrew Tacquard

    Andrew Tacquard

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    So as it gets cold velocity will decrease? I chronoed today, wife is on the computer so I don't have the data yet. I did not have as much margin as I wanted, but I did it with my 29. As soon as I get another pistol for my daughter to shoot I'll be back to my 20 for matches and that'll put me well in the safe zone. I think I had an extra 20 or so fps with it.
     
  20. fredj338

    fredj338

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    No, most powders are temp sensitive, losing vel & reducing pressures as it gets colder or increasing pressures & vel as it gets warmer. WST & probably a few more, are reverse temps sensitive. Meaning as the temps go up the pressures reduce slightly as does the vel. Read my blog on some testing I did.
    For making PF, IDPA or USPSA, build in a little margin. Everything affects vel/pressures; temps, altitude, even humidity, so if you are right at min, then you are risking a DQ. I run major @ 170PF +/-. That way I know I am good regardless of sea level or 6K ft, 40deg or 110deg.