Velocity Change in the wrong direction?

Turk10mm

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This weekend I was load testing and fired 50 rounds, 5 shots each, per .1 grain of powder.

I came up with an odd result. All rounds were loaded in the same sitting from the same batch of once fired fully prepped brass, that was also prepped in the same sitting. All shooting was done in the same sitting with no change in weather conditions. The chrono was never touched after initial setup and the gun was shot from a rest that never moved.

Varget Powder, Federal Brass, CCI BR2 Primers, SMK 168 seated exactly .010 off the lands.

44.0 Grains - Avg 2687 SD - 9.09
44.1 Grains - Avg 2703 SD - 8.17
44.2 Grains - Avg 2711 SD - 14.69
44.3 Grains - Avg 2736 SD - 10.00
44.4 Grains - Avg 2705 SD - 7.04

As you can see the standard deviations are very low. There were no oddball high or oddball low velocities in any of the 25 shots that made up the 5 averages above.

how in the hell did my velocity go from 2736 at 44.3 to 2705 at 44.4. The velocity dropped to the same average as 44.1 grains. My velocities consistantly climbed from 43.6 grains all the way to 44.3 grains and then suddenly dropped. 44.4 was the heaviest load I shot, so I don't know what would have happened at 44.5, unfortunately. That may have been a big teller that I didn't get.

Any ideas?

Thanks



43.6
 

shadow4

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Re: Velocity Change in the wrong direction?

Well, #1 you loads are only .1 of a grain apart. Your not going to see much change in velocity over that small of a charge. #2 you only have a small sample (5 shots). If you want a more accurate gauge send 10 to 20 shots over a chrony it will tell you more. #3, its possible that your chrony is not as accurate as you would like it to be.

FWIW, I Purposely reloaded some rounds .1 tenth high, and .1 tenth lower than the load I currently run to see if my anal powder weighing method was even worth the time. The All the .1 high loads, and .1 low loads ran the same average velocity of each other (basically they shot exactly same, .1gr up or down did not matter at all). BTW, that test was done over 30 rounds total. I could have done more rounds but it was telling me what I already needed to know.

Also by my calculations in my rifle .1 grain of a varget (if all other things are equal) averages about a 4 FPS difference in velocity for me. So even if I were +/- .2 tenths of a grain there "should" only be 12FPS extreme spread in velocity between the + and - rounds. If only it were that simple
wink.gif
 

Turk10mm

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Re: Velocity Change in the wrong direction?

Shadow, in my rifle, over 50 rounds I saw a definite 15-20 fps increase per .1 grain increase. This was true right up until I hit the last 5 rounds in my set, at the highest load I was shooting that day. I wish I'd had another load at 44.5 to see what happened there, but unfortunately I didn't..


They were very consistent. I know its not 10 or 20 rounds, but there's an obvious pattern of higher fps for the lighter charge.

44.3 2732, 2729, 2741, 2753, 2725 = AVG 2736
44.4 2711, 2698, 2696, 2706, 2714 = AVG 2705


 

Grump

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Re: Velocity Change in the wrong direction?

What "tolerance" is your Chrono supposed to be capable of? When they first became commonly available for less than thousands $$, I remember seeing a lot of "accurate to within .5%" (half a percent).

Some of the electronics wonks, IIRC, said they were really good for .25% or better for *precision* (repeatability of the same number for the same actual velocity) and the *accuracy* was usually was within .25% from the "real" number and "off" in the same direction for all measurements.

So, if all that is correct, your highest velocity of 2736 *could* be anywhere from 13.68 fps up or down from there (accuracy), and could be randomly within 7.64 fps plus or minus from the number reported. Let's round that to 7 even.

Avg 2687 could be anywhere from 2680 to 2694 if it's in the "middle" of the range.

Avg 2703 could be anywhere from 2696 to 2710.
This reading is 16 fps above the previous.

Avg 2711 could be anywhere from 2701 to 2718.
This reading is 8 fps above the previous.

Avg 2736 could be anywhere from 2729 to 2743.
This reading is 25 fps above the previous.

Avg 2705 could be anywhere from 2698 to 2712.
This reading is 31 fps below the previous.

So, the "noise" of the machine errors *could* put the real numbers for those last three as 2717 (15 fps above the previous), 2730 (13 fps above the possible previous), and 2712 fps (18 fps still below the possible previous, but far less of an anomaly). The "slop" could be even greater if the accuracy and precision really are 0.5%.

I think you have just found the middle of a nice load tolerant range, especially if the shots are all hitting inside the same "group" area from 44.1-44.4 grains of Varget with those 168s,

Just a thought.
 

Shady_Jay

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Re: Velocity Change in the wrong direction?

Generally when you see a drop in speed like that you are near an accuracy node. Notice your SD was the lowest in the speed drop.

How did that load shoot? What about the load after?
 

rijndael

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    Re: Velocity Change in the wrong direction?

    Is it possible you're varying the amount of time the round sits in the hot chamber.
     

    CoCaDoRi

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    Re: Velocity Change in the wrong direction?

    cooking the round...

    bearing length....

    look at the accuracy of 44.0 and 44.1 then check 43.9 for sd and accuracy if your sd is again a single digit, you found a good place to be...

    You can then load 44.0 if you are off a grn either way it is gonna shoot with low sd number.s
     

    Turk10mm

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    Re: Velocity Change in the wrong direction?

    44.4 was actually my best group, lowest SD, lowest ES. I just found it very odd that the velocity would drop.

    I timed all shots, 2 minutes between shots, 5 minutes between 5 shot groups. Its possible that it got to a threshold after 45 rounds, but who knows.

    I didn't get to shoot a load after, unfortunately..
     

    CoCaDoRi

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    Re: Velocity Change in the wrong direction?

    barrel length?

    might want to do a round robin test.

    load

    3 of each

    44.4, 44.7, 45.0, 45.3

    are you familiar with round robin tests?
     

    Grump

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    Re: Velocity Change in the wrong direction?

    Yeah, that's the sequence.

    Operating theory is that even a *regular* and non-random firing order will self-cancel out effects of progressive powder and barrel fouling, and possibly cancel out random shooter effects as well.

    Though it reduces the reliability of assessing precision with the very small sample size, I believe that limiting it to 3 shots has a corresponding benefit of reducing the chances of shooter error producing a flyer.
     

    CoCaDoRi

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    Re: Velocity Change in the wrong direction?

    yup one each (load) at it's own target...

    1 of 44.4 at a target ( dot )
    1 of 44.7 at its own target
    1 of 45.0 at its own target
    1 of 45.3 at its own target

    then repeat the sequence.


    You'll see a pattern develop...

    find the most accurate "target" then look at the data...

    if you feel you made an error repeat the process.

     

    CoCaDoRi

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    Re: Velocity Change in the wrong direction?

    I know a lot of br shooters that do this.
    You can find/learn a lot quickly in a sequence like this with a good chrony...
     

    Jerry m

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    Re: Velocity Change in the wrong direction?

    You said the weather did not change, but no location is given. Did the ammo heat-up or cool down before it was tested?

    Jerry
     

    Turk10mm

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    Re: Velocity Change in the wrong direction?

    Jerry, the ammo was 72 degrees in the house and taken outside where the low in the morning was 67 and the afternoon 72..

    Lumpy, brand new battery that day.. Less than 3 hours on time all day.
     

    Lumpy Grits

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    Re: Velocity Change in the wrong direction?

    Turk, do you test in the same place, shooting position and time each day? I'm making a SWAG here that it could be sun light issues and the Suns angle when you shoot.
    Respectfully,
    LG
     

    Fuzzball

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    Re: Velocity Change in the wrong direction?

    I don't know what the book "max" for that powder and bullet are and I'm not going to look it up. Actually, the book loads and what's best for our individual rifles vary way too much to make any book data a real guide anyway.

    Now, with that said, understand that when increases in powder charges stop producing increased speeds that is a common indication of excessive pressure. Why doesn't the over pressure make for more speed? Well, not only has the chamber pressure increased, probably a lot, but the burn rate has changed due to the pressure. It burns faster than normal, suignificantly changing the time:pressure curve. Basically meaning the excessive pressure is making the powder burn too quickly.

    Thus, flat or decreased speed during load development can be a good warning sign to back down a bit.

    Something for you to consider at this point.
     

    Country

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    Re: Velocity Change in the wrong direction?

    It could be the change in the light angle moving across the chronies screens as the day worn on.
    Try the last two loads again one after the other when the sun is high. If the velocity difference is in the right direction then its just the moving light and shadow upsetting the sensors.