The Purdy Prescription (PRX)

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PRX2500
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Joined: Mon Jun 26, 2017 9:22 am

The Purdy Prescription (PRX)

#1 Postby PRX2500 » Tue Jun 27, 2017 10:14 am

Good Evening All (At least its evening here in Kansas)

A friend gave me the heads up that he had read some interesting comments about PRX and tuning on this forum.

I would very much like to hear any results you may have had, both good and bad.

I would also like to answer any questions you might have about my tuning process.

PRX has been well represented by many of the Aussie RFBR shooters, and I have been interested if applying it to CF. I just don't have the equipment or facilities, so I have to rely on the help of others.

I look forward to hearing from you all

Thanks

Tony Purdy
The Purdy Prescription (PRX)
Does your rifle shoot "Purdy Good?"

jasmay
Posts: 1291
Joined: Fri Sep 28, 2012 9:26 pm

Re: The Purdy Prescription (PRX)

#2 Postby jasmay » Tue Jun 27, 2017 11:01 am

Tony, I have very likely been one of the first to employ your tuning method in CF in Australia, at least talking tk everyone on the forums and ranges it seems that way.

I'm not too keen on public discussion on this topic right now, feel free to PM me and I will chat with you, I can give him my contact details for a chat too.

Brad Y
Posts: 2181
Joined: Fri Jun 26, 2009 8:21 pm

Re: The Purdy Prescription (PRX)

#3 Postby Brad Y » Tue Jun 27, 2017 2:11 pm

Tony

I used a tuner on a barrel that had a bloop tube supposedly calculated on your formula. It worked well enough with the adjustable weight only needing to overhang the front of the tube a tiny amount. I would be keen to hear more on the subject and ho the calculations are done (by email or pm if you want to keep it quiet)

RDavies
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Location: Singleton NSW

Re: The Purdy Prescription (PRX)

#4 Postby RDavies » Tue Jun 27, 2017 7:49 pm

I also don't know much about the Purdy method but would certainly like to hear more about how it could pertain to centrefire

Old Trev-39
Posts: 289
Joined: Sat Sep 12, 2015 2:07 pm

Re: The Purdy Prescription (PRX)

#5 Postby Old Trev-39 » Tue Jun 27, 2017 8:16 pm

I have been using this method on both 6.5 &.284 "F" Class and 1000yd. B R. for around 3 years. Do not know the physics behind it, but believe it has something to do with how sound vibrates in a sealed tube. With my experimenting with this system I have found that the dia. of the hole in the extension or counter bore is a bit critical. Some where I picked up some info. that Obermaier had theory that the dia. for c/f calibres was .860 for 30 cal. .814 for .284 .757 for 6.5 .683 for .243 and .620 for .22 r/f. These are the sizes I have used and they seem to work. For me any how. I now fit this tune to any new barrel before I even start to shoot it.
Cheers,
Trevor.

PRX2500
Posts: 3
Joined: Mon Jun 26, 2017 9:22 am

Re: The Purdy Prescription (PRX)

#6 Postby PRX2500 » Wed Jun 28, 2017 12:01 pm

Hi All

It will be this weekend before I can go into much detail, sorry

If you have questions that you would like to ask me off line my email is:

prx2500@outlook.com

Have a good nite all

Tony

williada
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Joined: Wed Jul 22, 2009 12:37 am

Re: The Purdy Prescription (PRX)

#7 Postby williada » Fri Jun 30, 2017 1:03 am

Tony, thank you for contributing knowledge for others to build on. I have retired from public activities but I would like to make comment here in relation to CF if that is any assistance to you seeing your interest was brought to my attention by others.

It is a very interesting topic which Jason, Brad, Trev and I have discussed in the past, privately. I have written about Obermyer finding a bore diameter for 30 cal that worked as a bloop tube a few years ago. I also experimented many years ago with a ratio of caliber to tube bore diameter which worked. I called that the Obermyer ratio when I explained it to Trev. I notice now, which gives me further confirmation, the ratio of some commercial tuners produced in the States in recent times, were close to the Obermyer ratio. I tend to use those figures as a minimum bore diameter for each caliber.

It was Obermyer I believe, who thought flack and atmospheric conditioning behind the 30 cal boat tail bullet was important. He, like us was after performance at 1000 yards. We know harmonic tunes can be great with our F class rifles up to 700 yards with the barrel profiles we use if you can find one. We also know the vibrations are more complex in a centre fire with the interaction of three different moments of inertia and there is the torque factor which we tie to the weight of bloop tube or modern variable tuner extending beyond the muzzle if that is used, or if in the opposite case a back-bore in the barrel forms the tube with a thimble. Torque factor aside, there is still merit in just a back-bore to determine the approximate harmonic length if that is the only factor wanting to be determined.

There are three general classifications of tune which I consider, depending on the barrel’s behaviour which relates to stiffness or barrel lift and extreme spread. So the stiffer the barrel, the less we rely on harmonics for tune and more on SD’s for group. But hey if we could increase our “X” count.....your system can still refine things where in the past we used trial and error to establish harmonic lengths where they could be utilised. Some of us used to cut barrels back to find suitable length for accuracy. Some of us sit behind or in front of a node for conditions and distances in terms of positive compensation, so the perfect harmonic length is not a necessary condition for accuracy. Today consideration is also given to optimal barrel time and trade-offs to maintain high velocity for long range for certain conditions to maximise score which may not necessarily be representative of an harmonic node.

If we go down the harmonic tune path, I feel the Purdy system as I understand it, has great merit. There is nothing more satisfying than shooting tight harmonic groups, but note, not all barrels will be hummers and other tunes work for different distances or you can match the harmonic length of a barrel with load development without the need for an extension tube. But it is still a valuable option in the toolkit and is a part of the mix of things you do to get rifles to shoot. They work if velocity remains constant, but in Australia we can have four seasons in a day unlike the US which generally has more stable atmosphere.

If we link harmonic length to tuners, I feel some tuners could be improved by makers in Australia in recent times, claiming their product is based on the Purdy system.

It is my view the extension tube diameter based on the Obermyer ratio is a starting point in calculating a barrel harmonic length in Purdy terms as I understand them. The tuners one seller is promoting have used harmonic values suited to rimfire. They probably work for the wrong reason, as a damper of both torque and vibration rather than being approximate for harmonic length.

Where I have differed my application from the Purdy prescription that many others have followed by copying, without experiment, is the harmonic value which I apply is considerably higher as I explained to Denis on the forum when I gave my interpretation of the Purdy system when relating it to velocity. Trev has found the same I believe. Brad got a tuner made with a few more tricks included but its up to the pilot in the end to get the most out of their gear over the short and extreme distance we shoot in our competition.

David.

Brad Y
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Joined: Fri Jun 26, 2009 8:21 pm

Re: The Purdy Prescription (PRX)

#8 Postby Brad Y » Fri Jun 30, 2017 9:21 am

David

Its good to see you back posting, the forum was a lot less interesting without your posts.

RDavies
Posts: 2318
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Location: Singleton NSW

Re: The Purdy Prescription (PRX)

#9 Postby RDavies » Fri Jun 30, 2017 10:27 am

Do you have any links or calculations on how to work out what dimensions work best for a 31" long, 1.25" straight 7mm barrel?

williada
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Joined: Wed Jul 22, 2009 12:37 am

Re: The Purdy Prescription (PRX)

#10 Postby williada » Fri Jun 30, 2017 11:50 am

Rod, Mr Purdy may provide you with some information for strict harmonic length determination. At the moment I prefer to hold my information with regard to calculations for a number of reasons. One, the Purdy Prescription may form part of the calculation depending on barrel behaviour and secondly, I am about to get started on formal testing for proofs with the calibre you mention. That information will go to people who supplied new test barrels for them to determine what is to be released in consultation with me. I am supplying the ammunition. None of these things are cheap and I suppose we are moving beyond the scope of forum information.

There has been considerable delay to date because PTG has only recently supplied a custom reamer which I ordered last year. A base test barrel to compare with new barrels shot 58.3 with two pimples at 9 o’clock and 60.5 at 1000 yards last week under coached conditions in the Challenge Cup. The previous owner who you should know had re-chambered this old barrel and I asked him should I cut it down for the tests? He said no, with a smile on his face as the barrel had top scored again. This barrel has consistently produced the goods in last year’s Pennant. When the winter weather clears down south, I will begin in order to get meaningful information.

Rod, you don’t need help, you need a handicap lol.

williada
Posts: 969
Joined: Wed Jul 22, 2009 12:37 am

Re: The Purdy Prescription (PRX)

#11 Postby williada » Fri Jun 30, 2017 11:54 am

Cheers Brad,but I am going back into hibernation.

ajvanwyk
Posts: 440
Joined: Tue May 17, 2016 5:50 pm

Re: The Purdy Prescription (PRX)

#12 Postby ajvanwyk » Fri Jun 30, 2017 12:41 pm

RDavies wrote:Do you have any links or calculations on how to work out what dimensions work best for a 31" long, 1.25" straight 7mm barrel?


I don't understand much of it Rod but found the following calculation sheet that may assist you in your query. http://www.rimfirebenchrest.com/downloads/Purdy-PRX.xls

Albert
Albert
Rosedale Rifle Club
Australian Points Series

RDavies
Posts: 2318
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Location: Singleton NSW

Re: The Purdy Prescription (PRX)

#13 Postby RDavies » Sat Jul 01, 2017 9:36 am

ajvanwyk wrote:
RDavies wrote:Do you have any links or calculations on how to work out what dimensions work best for a 31" long, 1.25" straight 7mm barrel?


I don't understand much of it Rod but found the following calculation sheet that may assist you in your query. http://www.rimfirebenchrest.com/downloads/Purdy-PRX.xls

Albert

Thanks Alan, I have had a bit of a look and I think I will get one of my kids to work through this with me.

Old Trev-39
Posts: 289
Joined: Sat Sep 12, 2015 2:07 pm

Re: The Purdy Prescription (PRX)

#14 Postby Old Trev-39 » Sun Jul 02, 2017 8:00 pm

Hi Rod,
With my 6.5 and .284 I found that the 19th. and 21st. harmonic worked. You some times have to consider the overall length of the finished barrel and tuner for storage and transport. One 6.5 I had to cut a hole in the end of my gun case to carry it.
Cheers,
Trevor.

!Peter!
Posts: 150
Joined: Sun Oct 30, 2016 6:35 am

Re: The Purdy Prescription (PRX)

#15 Postby !Peter! » Mon Jul 03, 2017 3:12 am

My understanding of the Purdy Prescription is that it is based on tuning the acoustic resonance of a barrel.

My current belief is that the acoustic resonance is the sound travelling down the steel of the barrel but I'm not totally convinced. Anyway....

There are two ways to visualise acoustics. The first is as a wave as shown below:
Image
And the second is as molecular movement as shown below:
Image
Both of these representations are for the second harmonic.

The wave representation is easier to use in discussion but at a physical level the molecular representation is what is occurring in a cylinder.

It is one of my frustrations that when people post figures of a sinusoidal wave they don’t define what the x and y axis are. By not defining the axes it is not explicit what the wave is representing.

In the wave representation above the x axis represents distance along the barrel and y represents the pressure within the barrel at the x position, and the wave itself is at a point in time. The red line that is moving shows how the wave changes over time.

The wave and molecular representations above are what is seen in a cylinder which is open at both ends as shown below which shows the first 3 harmonics:
Image

However, sound traveling in a barrel, is being treated like what is seen in a cylinder which is closed at one end and open at the other end as shown below which shows the first 3 harmonics:
Image

Comparing open vs closed cylinder figures above you can see that the end of the barrel is located near an anti-node.

The objective of the Purdy Prescription is to add length to the barrel such that the end of the rifling is located at a node.

The equations for closed tube resonance can be found here https://en.wikipedia.org/wiki/Acoustic_resonance#Closed
I’ll leave it for you to link these equations to the Purdy Prescription equations or you can just trust me that is does :wink:

The main unknown in applying the Purdy Prescription is which harmonic to tune for…..


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