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Old 11-20-2020, 07:15 PM
Rimfireshooter99

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Purdy Calculation for a Harrell's Tuner



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If you have experience with the Purdy Tuner calculation method, could you help me with a tuner question? I just received a new benchrest rifle made by KSS, with a Harrell's tuner on it.

I have been working on using the Purdy calculation to determine the set point to start testing the tuner at the 9th harmonic. My issue is that the formula result at the 9th harmonic shows a physical length of 27.162 inches from the bolt face to the end of the inner tube. When I measure that on the rifle, I have 28 inches from the bolt face to the end of the inner tube.

Since I have no way to shorten the length to meet the Purdy estimate, my question is do I go on to calculate the 7th harmonic?

When I did that calculation, the bolt face to end of the inner tube distance is 28.178 inches, so I do have room to adjust the tuner shell out to the calculated distance.

I moved the outer shell to line up exactly with the edge of the inner shell, so they are both the same distance, my tuner is reading a setting of 218, which is approximately where it is expected to be.

Is this the correct approach to setting the tuner based on the calculation by Purdy?

Thanks for any clarification you can provide.
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Old 11-20-2020, 08:13 PM
vlnbyr

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Most good benchrest rifles will shoot good at a Harrells setting of 200 or so. Your rifle will shoot it's best at a setting somewhere between 0 and 500.
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  #3  
Old 11-20-2020, 09:12 PM
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Originally Posted by Rimfireshooter99 View Post
If you have experience with the Purdy Tuner calculation method, could you help me with a tuner question? I just received a new benchrest rifle made by KSS, with a Harrell's tuner on it.

I have been working on using the Purdy calculation to determine the set point to start testing the tuner at the 9th harmonic. My issue is that the formula result at the 9th harmonic shows a physical length of 27.162 inches from the bolt face to the end of the inner tube. When I measure that on the rifle, I have 28 inches from the bolt face to the end of the inner tube.

Since I have no way to shorten the length to meet the Purdy estimate, my question is do I go on to calculate the 7th harmonic?

When I did that calculation, the bolt face to end of the inner tube distance is 28.178 inches, so I do have room to adjust the tuner shell out to the calculated distance.

I moved the outer shell to line up exactly with the edge of the inner shell, so they are both the same distance, my tuner is reading a setting of 218, which is approximately where it is expected to be.

Is this the correct approach to setting the tuner based on the calculation by Purdy?

Thanks for any clarification you can provide.
Sounds like you have it correct, BUT
have you even shot the rifle without a tuner to get an idea of how it shoots? what too many do is just stick a tuner on and try to tune. you need a base line idea of how the rifle shoots.
also you need good shooting ammo or else you will be chasing ammo while trying to tune.

last thing you will be better off shooting 3-shot groups and adjusting 5-clicks out at a time.

Lee
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Old 11-20-2020, 10:44 PM
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There are MANY good threads on this subject and if you dig, you will find gold in each one.

First, find ammo that your firearm agrees with.

One important detail that took me a year to find, deals with obtaining an inner extension tube for the Harrells tuner from Killough and cutting it, to obtain your calculated length (expensive for what it is). Then, fine tuning by moving the weight in/out.

Take notes and have fun.
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Old 11-20-2020, 11:16 PM
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Originally Posted by KiddVolq View Post
There are MANY good threads on this subject and if you dig, you will find gold in each one.

First, find ammo that your firearm agrees with.

One important detail that took me a year to find, deals with obtaining an inner extension tube for the Harrells tuner from Killough and cutting it, to obtain your calculated length (expensive for what it is). Then, fine tuning by moving the weight in/out.

Take notes and have fun.
Right there it should tell you it has nothing to do with length. it is all about weight past the crown which affects barrel timing!
timing the exit of the bullet plain and simple

Lee
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Old 11-21-2020, 09:56 PM
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I've personally done around 15 or more barrels with Harrell tuners using the PurdyRX formula with excellent results each time (usually gets me within 30 to 40 clicks of final).
This is my experience, others may vary;
When I do the calculation I adjust the tuner for the distance off the muzzle using a vernier caliper. I think that's the only way to get any kind of precision.
There is only one point to all of this; You are NOT trying to find what ammunition your gun 'likes'. If you're going to do this there is no point in a tuner in the first place. You are trying to match the harmonics of the barrel to the MOST CONSISTANT ammunition, meaning chronograph stuff first to find the lowest extreme spread and lowest SD.
I tune at 100 yds because I think the spread at 50 is too small to see without going up to measure each group to see what, if any, progress is being made. Anything other than a wind free day (told by flags) is a waste of time and ammo. Rifle locked in a solid front and rear rest (padding moves too much).
I do 5 shot groups because I've seen too many "fluke" tight groups with 3. Your "average" groups will tend to "string" (a sign of the bullet leaving the barrel at different points of vibration and another reason for 5 shots). I move the tuner 10 clicks at a time (the stringing will usually get better or worse). If it gets better keep going until it gets worse then start at the best setting and move 5 clicks in either direction repeating the process, then 3 clicks etc.
Biggest mistakes I see people make is no controls of the experiment (rifle moves around too much, not paying attention to wind, etc).
Biggest laugh I get is seeing people test 4 or 5 different brands of ammunition without cleaning out the last brands lubricant from the barrel before trying the next brand. All the above is ONLY my opinion.
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Old 11-21-2020, 10:26 PM
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Originally Posted by jfleisc View Post
I've personally done around 15 or more barrels with Harrell tuners using the PurdyRX formula with excellent results each time (usually gets me within 30 to 40 clicks of final).
This is my experience, others may vary;
When I do the calculation I adjust the tuner for the distance off the muzzle using a vernier caliper. I think that's the only way to get any kind of precision.
There is only one point to all of this; You are NOT trying to find what ammunition your gun 'likes'. If you're going to do this there is no point in a tuner in the first place. You are trying to match the harmonics of the barrel to the MOST CONSISTANT ammunition, meaning chronograph stuff first to find the lowest extreme spread and lowest SD.
I tune at 100 yds because I think the spread at 50 is too small to see without going up to measure each group to see what, if any, progress is being made. Anything other than a wind free day (told by flags) is a waste of time and ammo. Rifle locked in a solid front and rear rest (padding moves too much).
I do 5 shot groups because I've seen too many "fluke" tight groups with 3. Your "average" groups will tend to "string" (a sign of the bullet leaving the barrel at different points of vibration and another reason for 5 shots). I move the tuner 10 clicks at a time (the stringing will usually get better or worse). If it gets better keep going until it gets worse then start at the best setting and move 5 clicks in either direction repeating the process, then 3 clicks etc.
Biggest mistakes I see people make is no controls of the experiment (rifle moves around too much, not paying attention to wind, etc).
Biggest laugh I get is seeing people test 4 or 5 different brands of ammunition without cleaning out the last brands lubricant from the barrel before trying the next brand. All the above is ONLY my opinion.
So we know what is excellent results in your opinion?

Lee
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  #8  
Old 11-22-2020, 07:33 AM
jfleisc

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So we know what is excellent results in your opinion?

Lee
1/2 MOA average
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Old 11-22-2020, 08:13 AM
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1/2 MOA average
100yds.?


Lee
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Old 11-22-2020, 09:04 AM
jfleisc

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100yds.?


Lee
1/2 MOA is the same for 50, 100, or 800yds. For those who don't know; 1/2 MOA = 1/2" @ 100yds, 1/4" @ 50yds, 4" @ 800yds
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Old 11-22-2020, 09:47 AM
linekin
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jfleisc
I believe you can find a tune in every revolution of a Harrell's tuner. Of course you'd be within 30-40 clicks using the Purdy prescription.
The only way to know if its the best tune for the rifle is using the Hopewell method & going back on another day & having repeatable results imo. OMMV
Not trying to be argumentative, just thats been my findings.

Keith
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Old 11-22-2020, 10:59 AM
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1/2 MOA is the same for 50, 100, or 800yds. For those who don't know; 1/2 MOA = 1/2" @ 100yds, 1/4" @ 50yds, 4" @ 800yds
Whatever tuning method is used, achieving consistent 1/2 MOA at 100 yards outdoors with .22LR would be pretty good as it's more challenging to achieve than consistent 1/2 MOA at 50.
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Old 11-22-2020, 11:18 AM
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jfleisc
I believe you can find a tune in every revolution of a Harrell's tuner. Of course you'd be within 30-40 clicks using the Purdy prescription.
The only way to know if its the best tune for the rifle is using the Hopewell method & going back on another day & having repeatable results imo. OMMV
Not trying to be argumentative, just thats been my findings.

Keith
I would tend to agree and resetting your scope day to day (condition to condition) is accepted norm. However, tuning before each match is unrealistic particularly without dead calm wind. Any other situation is just guessing.
And yes, there is a 'different' tune for each brand of ammunition however, once again, trying to tune for inconsistent (poor quality) ammunition is a waste of time and the money spent on the tuner.
My assumption is the Purdy equation presumes a somewhat narrow velocity range (standard velocity so as not to run into the transonic issues of hi-velocity). But, as you say, it can't make up for differences in lubricant for instance. The bottom line is that using the formula has saved me hundreds of dollars in match ammunition trying to shrink groups.
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Old 11-22-2020, 11:28 AM
jfleisc

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Whatever tuning method is used, achieving consistent 1/2 MOA at 100 yards outdoors with .22LR would be pretty good as it's more challenging to achieve than consistent 1/2 MOA at 50.
Yes, I agree, time in flight and wind being what it is. I would like to clarify what I constitute as "consistent 1/2 MOA. Meaning 5 shot groups at 1/2" (at 100yds) or better more than half the time not just once or twice (even a blind squirrel finds a nut sometimes). For instance; just yesterday while function testing I let another young shooter try 3 shots with my rifle (he is used to shooting semi automatics). He shot a 4/10" group @ 100yds but I would bet against his repeating it.
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Old 11-22-2020, 06:30 PM
Hi-NV Shooter
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1/2 MOA is the same for 50, 100, or 800yds. For those who don't know; 1/2 MOA = 1/2" @ 100yds, 1/4" @ 50yds, 4" @ 800yds
Wow your .22lr can do 4" groups at 800yds!

Not 800yds. but at 100yds. not to bad from my experience
10-shot group 100yds.

Both targets shot under a watchful eye of a Match director. the rifle that shot them was tuned at 50 yds. using 3-shot groups and 5-clciks at a time.

So again every rifle you used the PRX can do 1/2" MOA ?

Lee
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