So, this is interesting...

Discussion in 'Reloading Room' started by Branth, Feb 17, 2015.

  1. Branth

    Branth Member

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    I'm loading 9mm 124gr Bayou bullets with Bullseye, trying to find a load that makes IDPA power factor. I start out with a mild starting load and get around 950 FPS. I boost the load by 0.2gr, and get up to 975 fps. "Oh, cool," I say to myself, "It looks like I gain around 25fps when I bump the powder charge up by 0.2gr. I'll just bump it up another two and I'll be right around 1000fps, pretty close to Power Factor, and when the weather warms up, I'll be above the threshold easily!"

    So I do this, and... 1040 FPS! Somehow, that additional 0.2gr powder gave me an extra 65 fps. I'm wondering if the pressures just got high enough that the powder burned more efficiently or something.
     
  2. Or the pressure-time curve shifted to the point that it made for a more favorable condition.
     

  3. More powder equals less free space in the case, also minor changes in fast powder make significant difference. Much more than lower pressure levels. Increasing powder at the lower end will have mild affect, increasing on the top end has dramatic affect.
     
  4. Branth

    Branth Member

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    So, they say there are warning signs of high pressure you can pick up with a chrono... Want to school me?
     
  5. Mostly by looking at published data for the bullet, and the powder. Velocity is usually listed for maximum loads. Some books or data also list the pressure for maximum loads.

    Other signs to look for are excessive bulging of the cases on semi auto. Extreme flattening of the primers, or in extreme cases the primer flowing back into the firing pin channel.

    It wouldn't hurt to keep some factory fired cases to compare primers and measure the bulge. Your loads with new or once fired brass should not exceed the average from several spent factory casings.

    I highly suggest using new brass for pushing the limits, or once fired only after running them through a bulge buster. Also cases stretch forward after firing and being resized, you can take measurements before and after of the stretch of factory ammo. Then do the same with handloaded ammo, but the difference will take a very sensitive micrometer.
     
    Last edited: Feb 17, 2015
  6. Branth

    Branth Member

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    I'm good with the brass signs, but I seem to have read of other methods that needed a chrono. Something about linearity, though I don't know exactly what.

    As for velocity, that data would only be valid for their specific barrel length and action, right? I don't own a 22" barrel, but every powder company seems to.
     
  7. ajole

    ajole Supporting Member

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    You've already found the point where linearity was lost.

    Linear data says, if you change the powder by X, then the speed should change by Y. Add X again, speed increases by Y again. But you added X, and got Y+...that's not linear.

    Something in the equation went wonky, instead of linear, you are going exponential...that's bad.;)
     
  8. I won't get exited about it being over the pressure limit, if there are no signs on the brass. Plus your fps is less than what is listed by some and in the range of what others list.

    If you want to really know what the pressure is, you spring for pressure testing equipment. The equipment is not that expensive and you'll never be sorry you bought it.
     
  9. noylj

    noylj Member

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    Did you fire ten shots to get a reasonable average or one shot? One shot tells you almost nothing.
    If you fired ten shots at each charge weight, you would probably have seen the lowest velocity at the upper charge was within the range of the lowest charge.
    Statistics is very important.
    Another things is that whenever you weigh a charge, and the balance is actually sensitive enough, your charge of, say, 3.3gr is really somewhere between 3.25 and 3.35gr. So, shooting one shot per charge, you may have fired 3.25gr, 3.45g, and 3.75gr. Again, that gets sorted out with a decent statistical sampling.
     
  10. Branth

    Branth Member

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    So, powder charge vs velocity is linear until pressure gets too hot? Or does it do an S thing and then start dropping off when it gets too hot?

    I know the difference between linear and not linear, but I just don't know what indicates overpressure, whether it be curving upwards and getting more velocity or curving downwards and getting less. I read a few places where it seemed to indicate curving downwards.

    Yeah, I took at least 10 shot samples with each of these. Most of them I have at least 2 10-shot samples. I've taken a few statistics classes, and I'm trying to apply that to reloading.

    One thing I've been doing is instead of just using the standard deviation as given by the chrono, I'm calculating it myself, and using the sample standard deviation instead of the plain population standard deviation, since I want the standard deviation of all ammo fired from the gun and not just the sample I took. I'm also converting these values to a relative standard deviation (the standard deviation as a precent of the whole), which is more relevant for comparing loads of very different velocities - A standard deviation of 20fps is really small for a 2700fps .308, but a lot bigger for a 9mm round moving at a leisurely 1000fps.
     
  11. ajole

    ajole Supporting Member

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    It might (should) be the excess pressure that starts ramping up the speed in a non linear way.

    But are you asking about the rate of velocity gains being less steep despite more pressure, like a diminishing return, or about velocity actually dropping off, and going slower as pressure keeps building?

    Velocity is theoretically directly related to pressure, but velocity is not a good indicator of "over pressure" as there are too many variables to be sure that really fast bullets are, in fact showing over pressure. Weights, diameters, jacket material, crimp, seating depth...all could affect pressure and/or velocity, in either direction, but how much velocity indicates too much pressure for the gun, or the case?

    Over pressure would be more easily and reliably observed in the brass and the primers.
     
  12. Branth

    Branth Member

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    I guess what I'm asking is whether ramping up or ramping down indicates overpressure.
     
  13. You really can't count on that without doing batches with the same brass, same primer, and a good number for an average between lots.

    It is not unusual to have a wide spread with the same load. Powder is very finicky, and scales to not measure into the hundreds, just tenths. You could easily be off .1 grain from one case to the next.

    I suggest buying a caliper, and a micrometer if you do not already have them. They would be the best indication of over pressure. If you are looking for standard pressure take the measurements from standard factory loaded ammo. If you are looking for plus P take the measurements from plus P ammo.

    Always do averages on ammo batches, do not rely on only a couple rounds. I work in batches of 5, then when I get to my goal batches of 10, if that is consistent I will do another ten. So for the final load I will use at least 25 rounds, not counting the rounds working up to the goal.

    And use a good scale to measure every single load while working up. DO NOT COUNT ON A AUTOMATED POWDER MEASURE! If you are not pushing limits then it is fine not to weigh every load, but your velocities will be even more inconsistent.
     
    Last edited: Feb 19, 2015
  14. noylj

    noylj Member

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    Charge weight to velocity is "generally" linear (in that the best fit of the data is a straight line and not that each avg. velocity reading is directly on that line. Over pressure, for some cartridges like .44 Mag, can be seen by the velocity suddenly taking a noticeable dip as the charge goes up. This is an INDICATION of possible over-pressure. IF one knows that they are working up to max load and know that they are getting the approximate velocity for a max load, one can take the velocity dip as a good sign of over-pressure. One can also measure the case pressure "bulge" about ¼" above the extractor groove or (when dealing with really high pressure like a rifle) the actual case head expansion as one works up as another indicator of pressure.
    Someday, we will hopefully have pressure transducers built-in and KNOW what pressure we are running.
     
  15. Hot-Shot

    Hot-Shot Member

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    I've done some fine tuning by shortening OAL (WITHIN LIMITS) I use mainly tight group a fast burning powder x.x gr.with fp 125 gr exteremes

    Edited to remove load data
    -Branth
     
    Last edited by a moderator: Feb 20, 2015
  16. Branth

    Branth Member

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    Sorry, but we don't allow load data to be posted on the forum for liability reasons. I edited it out.
     
  17. Your 'INDICATION' and 'IF' certainly are correct.

    Until then, the Pressure Trace 2 system using strain gages is certainly best option out there for the average Joe. It's only $500 and you'll never be sorry you bought one.

    http://www.shootingsoftware.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=RSI&Category_Code=PT
     
  18. Branth

    Branth Member

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    I ran into that the other day. That whole website just about gave me a nerdgasm. I think I'm gonna need one.
     
  19. Branth

    Branth Member

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    Tested another load today. I increased 0.1gr above the 975fps load (halfway between that and the 1040fps one) and got 1008 fps. All of these velocity values are averages of at least 10 shots. Some are averages of 20 shots.
     
  20. noylj

    noylj Member

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    With the .44 Magnum, I found that the first sign of exceeding max was a drop in velocity. However, that was based on the "best fit" straight line and not that every charge/velocity was exactly on the line.
    124gr lead bullets and Bullseye can be at max with a velocity from 990 fps to 1175 fps, depending on the gun and the exact lot numbers of components used.
    950 fps is above the expected velocities for starting loads I have.
    If one uses the case bulge as an indicator of pressure, I prefer to use virgin brass of the same make/head stamp as the factory ammo I use for comparison. Mic all around the case to be sure you have measured the bulge.