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Thread: Speed decreases with more powder

  1. #1
    Boolit Master Revolver's Avatar
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    Speed decreases with more powder

    I've been working on a load for my S&W 500 using a 375 grain spire point and Accurate 1680 powder. FPS kept rising until my last batch, it went down. The last two loads were compressed, or pretty close to it. Is this normal?

    38.5gr 1236
    40.0gr 1337
    40.9gr 1360
    41.5gr 1351

    Also today, testing some 308 loads with benchmark powder the speed quit rising from 38.0 to 39.0 grains...

    36.1 2233
    37.0 2264
    38.0 2381
    39.0 2382

    My method is to chrono 5 shots, delete one wild shot if exists, then average the remaining 4 shots. I don't know if that's a good method, but it seemed reasonable to me.

  2. #2
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    Assuming your data was sufficient to be statistically meaningful, yes.

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  3. #3
    Boolit Master

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    You need an absolute minimum of seven valid data points per load to establish trend. It's been a long time since I took statistics, but I think 37 is one of the magic numbers for group trend. Rather than average, figure the mean.

    The effect you observed in your .500 is a result of load density behavior that some powders exhibit. I've seen it with 2400 and Reloder 7 in straight-walled cases under heavy boolits, right at the point of compression the velocity either levels out or declines. Some powder/cartridge/boolit combinations will dangerously spike pressures at the point where compression begins to take place, others not so much, so be very careful drawing any "certainties" from your observations.

    The effect in your .308 is one of the methods some folks use for load workups, sort of akin to the "ladder method", where you begin with a starting load and increase the charge in consistent, small increments until the point of impact levels stops rising for two or three loads, indicating a velocity plateau. The "sweet spot" for accuracy and consistency throughout a broad temperature range may be within that window. I have a pretty strong opinion that it is more a function of barrel harmonics than powder behavior in bottlenecked rifle cartridges.

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  7. #7
    Boolit Bub
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    I see velocity increase in some of my black powder loads that were under to much compression. When I backed off in powder but kept the compression die the same, velocity increased and the es and sd are better also.

  8. #8
    Boolit Master
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    Velocity decreases when maximum has been reached at times. Dont now if that applies here. I didnt check your data to max.

  9. #9
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    I use this as a warning sign in mid capacity cartridges such as the 7BR and many other such cases with mid to slow powders while increasing charges by 1/2 grain, tells me . . . ok dummy, that's far enough back down. Dunno if it applies to larger volume cases such as the 308. In the mid capacity cases it's where accuracy, consistency of the SD and velocity head south.

    I use nothing less than 10 shot chrono tests and that only gives me a reasonable idea of consistency. I throw out none of the shots fired in a test, that one shot could well be trying to tell you something about the consistency of the load. Actually it's probably telling you a great deal about consistency.

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  10. #10
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    I've had that happen in more than one bottle-necked rifle cartridge.

    To me, that signals that I've reached to pressure limit for that powder, case, bullet, and primer combo. If you add more powder, the pressure climbs, but the velocity does NOT!

    Measurement of the pressure ring just above the extractor groove will tell you the internal pressure has gone way above the safe pressure point for that powder. Usually the primer pocket also expands. Increases of just .0002 to .0003 is too much. It tells you have permanent expansion of the case head.

    Shade tree reloaders use that as a poor mans pressure gauge. It has been said to measure a factory load using similar bullet weights. Because factory loaded ammo is mostly loaded to max pressure,(so the published velocity is reached.) When your handloads get to the factory measurement, you're pretty close to max pressure.
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  11. #11
    Boolit Master







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    Agree with Hickory, have also seen it more than once.
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  12. #12
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    and a velocity decrease DOES NOT mean a pressure decrease or plateau.
    it's all an educated guess,,,, till the trigger is pulled.

    the more i find out about shootin boolits, the more it contradicts everything i ever learned about shooting jaxketed.

  13. #13
    Boolit Master



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    Quote Originally Posted by snuffy View Post
    Shade tree reloaders use that as a poor mans pressure gauge. It has been said to measure a factory load using similar bullet weights. Because factory loaded ammo is mostly loaded to max pressure,(so the published velocity is reached.) When your handloads get to the factory measurement, you're pretty close to max pressure.
    Ken Waters wrote for years in Handloader that this was his method of determining max pressure. Didn't like this method myself, not all cases will expand the same with the same pressure in them. You could have one case that says all is well and the very next case is saying uh-oh.

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  14. #14
    Boolit Master



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    Compressed charges of some powders don't ignite as well as non compressed charges thus giving less velocity.
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  15. #15
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    ahh,, while others want compression to burn more efficiently.

    airc ken also recommended pulling down a few rounds from the factory ammo and using the cases for your load to compare the CHE it was supposed to be an apples to apples comparison of the brass.

    near velocity and near CHE measurements should equal near pressures.
    it's all an educated guess,,,, till the trigger is pulled.

    the more i find out about shootin boolits, the more it contradicts everything i ever learned about shooting jaxketed.

  16. #16
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    The more powder you have the more mass of gas along with the bullet that has to go out the muzzle.
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  17. #17
    Boolit Master


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    When you see a diminishing velocity increase per grain of powder you've reached the limit of that powder/barrel combination.
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  18. #18
    Boolit Master




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    Quote Originally Posted by geargnasher View Post
    Y Rather than average, figure the mean.

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    Oh, Criminy, Gear - the Average IS the Mean! Good grief...
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  19. #19
    Boolit Master

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    Revolver, see why this is the best place? You get straight answers.
    What you see can happen with ANY cartridge. More pressure does not mean more velocity.
    But do not toss wild readings, it means something is wrong. Yeah, sometimes the chrono will not pick up the boolit, just gas so that is different.
    One shot with a high spike in velocity or lower is an indication of ignition problems. Could be the primer or the powder you are using. Powder compression will cause ignition problems.

  20. #20
    Boolit Master Revolver's Avatar
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    Thanks for all of this info, I have tried to integrate some of it into my collective (brain).

    Utilimately it seems that I've made a booboo. The 308 info was for a cast boolit and now I see read that I should keep them under 2k fps.

    36.1 2233
    37.0 2264
    38.0 2381
    39.0 2382

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
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