TESTING BULLET WEIGHT VARIATION VS ACCURACY

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  • Last Post 21 January 2018
joeb33050 posted this 19 January 2018

 

1/19/18

 

My records of cast bullet weights, for 26,948 bullets, shows the standard deviation average at .155 grains. A skilled caster should be able to produce 180-210 grain bullets with a standard deviation of about .125 grains; perhaps putting some outliers aside.

 

Then 68% of the bullets weigh +/- .125 grains, 95% weigh +/- .25 grains, and 99.45% weigh +/- .375 grains. 55 of 10,000 would weigh outside +/- .375 grains.

 

The questions then become:

 

Are groups shot with 180-210 +/- .4 grain bullets larger than groups shot with 180-210 +/- 0 grain bullets?

 

If so, are groups shot with 180-210 +/- .3 grain bullets larger than groups shot with 180-210 +/- 0 grain bullets?

 

If so, are groups shot with 180-210 +/- .3 grain bullets larger than groups shot with 180-210 +/- 0 grain bullets?

 

(N.B. +/- 0 is in fact,~ +/- .05 gr)

 

If group size differs by 10%, it seems that about 55 5-shot groups with each bullet, 110 groups or 550 bullets, are required to be reasonably confident of the difference. My guess is that this ain’t going to happen.

 

Instead, 5-shot groups with 4 bullets +/-0 gr and 1 bullet +/- some value, +.4 gr for example, should fairly quickly give us a notion of whether small bullet weight variations affect accuracy. This method can be used easily and inexpensively.

 

joe b.

 

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Ross Smith posted this 21 January 2018

Joe: What kind of scale are you using?

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joeb33050 posted this 21 January 2018

RCBS

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joeb33050 posted this 21 January 2018

RCBS 10-10

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RicinYakima posted this 21 January 2018

Joe, Received the package of bullets. My plan is to do a series of static comparison test for shape, then dynamic tests for center of mass vs. center of form. If there are no interesting observations, then I will start milling them apart. Any other suggestions? Ric

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joeb33050 posted this 21 January 2018

Ric, Inaccurate 22 jacketed bullets for 30 years or so have had cannelures and are fmj with open, lead visible bases.

Latest tests show ACCURATE jacketed fmj with open, lead visible bases, so it ain't the fmj or bases.

The bullets vary widely on weight, but selected same-weight are not accurate.

Some characteristic causes inaccuracy. Don't know what.

Thanks;

joe b.

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joeb33050 posted this 21 January 2018

Joe: What kind of scale are you using?

I have the weighing records in an EXCEL file, happy to send it to anyone. PM your e-mail. 

joe b.

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Ross Smith posted this 21 January 2018

Thanks Joe, I have an old balance beam also. I tried one off brand electronic scale, couldn't get the same reading twice.

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RicinYakima posted this 21 January 2018

Ross, it is a struggle! I working on joeb's bullets today. My electronic scales reads to 0.01 grain, or one hundredth. Have to shut heat off in room, turn off florescent lights and use LED's. Put bullet on scale; slowly move hand back and wait 20 seconds. The air currents of moving my hands causes fluctuations in air around pan. It is not fast, but better than the old balances in the Chemistry labs of 50 years ago.

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