The Purdy Prescription

B

Brian_K

Guest
Not sure if all are aware, but a guy called Tony Purdy has come up with a method of tuning rimfire rifles which appears to be having some good results.

I asked Tony if he though his system could be applied to air rifles, he reckons so and has given me a new formula to work to.

I list it below for those who may be interested:

"With the slower speed of the pellet and the milder vibrations, I am wondering if working in the lower harmonics will have a better chance of working

What that means is that any length we work with past eh muzzel will be a bit longer. But being that the air rifle barrel tends to be on the short side, this might not end up being to bad

I understand that AR barrels are a bit finicky when it comes to tuning, so lower harmonics may be of benefit as well.

If someone had a 16" barrel and we use the 7th Harmonic. lets see what we would need for a length

There are 7 quarter waves in a tube for the 7th harmonic, so we want the pellet to leave at the 6th QW

divide the barrel by 6 16/6 = 2.667

Since we need 7 QW to have the correct harmonic multiply by 7 2.667 X 7 = 18.667 This is what is called an acoustic length

The waves in a tube do not rebound directly at the end, they travel a bit past the end before reflecting. This is part of the acoustic length and needs to be removed to get the actual physicallength to have a barrel tuned to the 7th Harmonic. This is called an End Correction

I have found that using .3 times the diameter of the tuner opening gives us the best EC. If the opening of the tube is 10mm, we would multiply 10 X .3 and we get 3mm.

We will need a tube/tuner to get the length we need to tune

If we have a tube that is 2.667 and we remove the 3mm ( sorry for the mix of measurement units) we come up with a physical length the tuner needs to be. Now all you need is a tube thick enough to support being bored to fit the barrel, with a tuner bore of 10mm, and long enough to attach to the barrel and still have the correct distance out past the muzzle."

Using my 17" barrel on the TM1000 I came up with a figure of 19.644" and have made a tube out of brass to give me the required length. See how it goes this weekend.

Brian
 
Finished and set to 19.644” (or as close as humanly possible).

Hopefully get time to try it tomorrow.

tuner_zps0b84abc1.jpg


I see a problem with muzzle blast in the tube upsetting the pellet???? so maybe some vent holes will appear in the future


Brian
 
Would this be why when one get the speed with a silencer fitted "just right" then one can achieve super accuracy (even more than with a bare barrel) ??

All my rifles are "multi purpose" and there fitted with silencers. I've noticed over time though that all barrels with the silencer have a definitive "sweedspot".
 
mount

Brian_K said:
Finished and set to 19.644” (or as close as humanly possible).

Hopefully get time to try it tomorrow.

tuner_zps0b84abc1.jpg


I see a problem with muzzle blast in the tube upsetting the pellet???? so maybe some vent holes will appear in the future


Brian
Click to expand...
How are you mounting it to the barrel?
 
Turboman said:
How are you mounting it to the barrel?
Click to expand...

I made it a tight sliding fit (need to tap it with a block of wood) then just the one 5mm grub screw.

Never got the chance to try it today (grandkids came round), hopefully tomorrow night.

Came up with a simple spreadsheet if anyone wants to use it, just add your own values for barrel length and diameter: https://dl.dropboxusercontent.com/u/53488303/Purdy%20Tuner.xls (for air rifles only)

Tony is checking one out that will allow for metric and imperial sizes and will calculate the value for all the harmonic waves.

Brian
 
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Brian_K said:
I made it a tight sliding fit (need to tap it with a block of wood) then just the one 5mm grub screw.

Never got the chance to try it today (grandkids came round), hopefully tomorrow night.

Came up with a simple spreadsheet if anyone wants to use it, just add your own values for barrel length and diameter: https://dl.dropboxusercontent.com/u/53488303/Purdy%20Tuner.xls (for air rifles only)

Tony is checking one out that will allow for metric and imperial sizes and will calculate the value for all the harmonic waves.

Brian
Click to expand...

Downloaded the spreadsheet. A couple questions. When measuring barrel length is it from the breech, right where the pellet inserts? On my Steyr the barrel is turned down for a short distance where you load the pellet. Or should it be measured in terms of unsupported length? Also, for this formula, this assumes whatever you machine for your barrel, the exit hole in the tuner is 10mm....no narrower, no wider? So on a 16mm barrel the part that mounts would be bored to 16mm ID then stepped down to 10mm ID past the muzzle? Does the OD or material of the tuner matter? Guess that was more than a few questions :)

-Casey
 
DoctorBoudreau said:
Downloaded the spreadsheet. A couple questions. When measuring barrel length is it from the breech, right where the pellet inserts? On my Steyr the barrel is turned down for a short distance where you load the pellet. Or should it be measured in terms of unsupported length? Also, for this formula, this assumes whatever you machine for your barrel, the exit hole in the tuner is 10mm....no narrower, no wider? So on a 16mm barrel the part that mounts would be bored to 16mm ID then stepped down to 10mm ID past the muzzle? Does the OD or material of the tuner matter? Guess that was more than a few questions :)

-Casey
Click to expand...

Hi Casey,

Yes, the barrel is measured from the breech face (across the flat face).

The 10mm Tony mentioned is a little confusing, he used this as an example barrel diameter (he’s not an air rifle guy). As you, my barrel is 16mm so that’s the exit diameter I used (I checked this point and it’s OK).

As far as the outside of the tuner sleeve goes, I just went with what looked right (to me) and ended up with a diameter of 23mm, again I checked this with Tony and that was fine.

So far I have only had one very short test with this system, I wasn't able to determine conclusively if there were any benefits, but neither were there any detrimental effects, so a lot more testing to do.

Brian
 
I did the calculation today on my EV2

I used the 9th harmonic so will re-calculate for the 7th. The EV2 comes with a nice long air stripper that can be positioned, via a set screw, to any place in it's scope. Using the 9th and the size of the hole in the Stripper, I would only need to have 2.409" of the stripper hanging beyond the muzzle. I can see now why, when I was fooling with the Stripper with a movable slide it was so finicky. I'll re-calculate and see what I find. Fun stuff, eh? I will be able to test tomorrow.

Pete

P.S.

My barrel is 1 a bit over 19". Should I use the 8th or do you think the 7th is right?

Thanks,

Pete
 
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I think stick with the 7th Pete and maybe send me your findings of the test, some pictures of the group sizes and your observations would be useful. You could send direct to Tony or myself and I’ll pass them along. If you have more time then certainly try the 8th & 9th and send those as well.

I’m having a long test session tomorrow and will do the same.

I reckon the more information he has the more accurate his system will become.

Edit: sorry I missed your ps – as I understand it, it’s the speed of the pellet which determines which harmonic wave is chosen, not the actual size of the barrel. I’ve asked Tony to chip in on this thread as I’m now at the limit of my knowledge (in fact I passed it a while ago :confused:)

Brian
 
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Hi All

Brian suggested I check in here. I have to apologize, I have been a bit overwhelmed in the RF forums to see much past that. I appreciate all the work Brian has done.

I will add this to the list of my daily checks to see if I can help. I am very interested in seeing this work with air rifles. It may require a bit more precision in the calculations, but it sure will be fun trying to get it right.

Thanks for having me

Tony
 
No Joy in Mudville :(

I tested today in my big garage. I shot the EV2 a bunch and could not find anything consistent, using the formulas for the 7th, 8th or 9th. I found the same touchy conditions I had previously found with the sliding tube stripper a couple of years ago. I didn't have time to verify with my mid-barrel tuner but will do, perhaps tomorrow. A setting that would give a single small hole would turn into vertical when going beyond 5 shots.

Pete
 
Hi Peter

I am thinking that we need to go the other direction of 3, 5 or 11th(this will be an odd tuner but works very well for the RF)

Calculations are the same for the 3 and 5. 11th you need to divide by 8 then multiply by 11.

The lower harmonics are "slower" moving and may be more conducive to tuning.

Have a great nite

Tony
 
Pete Wass said:
I tested today in my big garage. I shot the EV2 a bunch and could not find anything consistent, using the formulas for the 7th, 8th or 9th. I found the same touchy conditions I had previously found with the sliding tube stripper a couple of years ago. I didn't have time to verify with my mid-barrel tuner but will do, perhaps tomorrow. A setting that would give a single small hole would turn into vertical when going beyond 5 shots.

Pete
Click to expand...

Hi Peter

Interesting that the vertical appears after 5 shots. Have you seen this issue before in other tuning. Was there a solution?

Need to learn this stuff. I do not have an air rifle to work with.

This is very interesting

Tony
 
Yes, I have

PakRat said:
Hi Peter

Interesting that the vertical appears after 5 shots. Have you seen this issue before in other tuning. Was there a solution?

Need to learn this stuff. I do not have an air rifle to work with.

This is very interesting

Tony
Click to expand...



It has been my experience with rifle tuning that one can get a fantastic, tight group and then throw shots either vertically or in any direction. I once bought a CF Bench Rifle from a friend who could not make it shoot consistently. I was sure I could make it shoot so I bought it from him. Once at the range, I shot a three shot group with it that measured less than the size of the bullets but the fourth shot was 1/4" away from that group. Sold the rifle to a buyer who used it for a less accurate game.

Another issue I believe I notice is extreme temperature swings seems to raise havoc with rifle tune. It will be interesting to see if this harmonic tune is less or not at all affected by temperature fluctuations. Often along with temperature change comes humidity change, which is believed to de-tune rifles. Density Altitude readings have been used by some to keep rifles in tune when they move the tuner along with the DA changes. Lots of fun ahead I think.

Pete
 
Thanks Tony

PakRat said:
Hi Peter

I am thinking that we need to go the other direction of 3, 5 or 11th(this will be an odd tuner but works very well for the RF)

Calculations are the same for the 3 and 5. 11th you need to divide by 8 then multiply by 11.

The lower harmonics are "slower" moving and may be more conducive to tuning.

Have a great nite

Tony
Click to expand...

I will try the 3rd, 5th and 11th , probably next week.

Thanks

Pete
 
And the winner is - - - -

The 5th. I made an aluminum bloop tube out of 3/4" and long enough to cover all the scenarios. I began with the 5th. I set the tube to length at home and then went to my range and tried it. Dang, it was close so I moved it just a few, mebby 5 thou and one hole! There was a fair amount of wind and but I got 5 pellets into one small hole. I then shot some bulls on the target I was using to test. I could smoke the x rings when I paid close attention to the flags. The rifle shot predictably. I even tried a pellet with a damaged skirt and right into the same hole that I had made with the pellet before. And this was with some pellets that hadn't shown much promise before. This is the best my EV2 has shot to date.

I like it :) .

Pete
 
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Pete,

Can you show us your math for the 3rd harmonic? Did you divide by 8 and multiply by 11? Thanks.
 
Welllll - - - -

I just went through my notes and it looks like I set up on the math for the 5th harmonic. 19.292 / 4= 4.823 x 5= 24.115 AL. 24.115 -.240=23.875, PL. I think I started here because this was the shortest in length. Now I am wondering if I should cut the bloop tube down so the clamping portion is closer to the muzzle. The tube extends 4.710 beyond the muzzle leaving 6 3/4" of tube and clamp back on the barrel.

I'll edit my other post.
Pete
 
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Do these calcs look correct for a 17" barrel (5th harmonic)?

5th harmonic.jpg
 
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