The Purdy Prescription (PRX)

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plumbs7
Posts: 1124
Joined: Wed Feb 12, 2014 7:32 am
Location: Dalby/ Tara Rifle Club

Re: The Purdy Prescription (PRX)

#16 Postby plumbs7 » Tue Jul 04, 2017 12:40 am

!Peter! wrote: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:
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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…..

This will be my last post before being deregistered at my own request .
So I thought I would show what I believe is another way to modify harmonics . I do get a lot of flack about it and even on my own F b page they are challenging me to do more testing and publish!
Ok , what does it do ? It's passive tuning and comes under the same class as mass weight dampeners!
Tuners are active dampeners and in the right hands can be extremely effective between changing conditions!

What do orings do ??? It's very much like your shock absorbers on your car going down a rough road . It calms them down and extends the nodes ! This method was first used by RF benrest I believe!

It also seems to aid in slight positive compensation and has shown flat waterline at the longs . This is consistent with my historical results , as I seem to do better at the longs . And this is over a number of different barrels . There is a trick in tuning a barrel with orings . But once set , you should just forget and leave them .

But basically the main orings are the ones in the middle and the secondary's are roughly 4-6" from the muzzle .
It's no magic bullet , in that you still have to concentrate on barrel timing and optimum charge weight.
It is also doesn't work on every barrel or help u to wind read any better !
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After tonight I will not be able to reply or contribute. Hence why I am giving up one of my secrets!
Regards Graham.

sungazer
Posts: 189
Joined: Fri Mar 17, 2017 7:58 pm

Re: The Purdy Prescription (PRX)

#17 Postby sungazer » Tue Jul 04, 2017 5:03 pm

Harmonics are created by the fundamental wave and you are discussing what happens in the barrel in the second, third harmonic or even could be termed standing wave as in radio terminology. The bullet however is traveling supersonic faster than the fundamental wave that creates the harmonic. I am yet to be convinced.

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

Re: The Purdy Prescription (PRX)

#18 Postby PRX2500 » Wed Jul 05, 2017 10:40 am

Hi All

Sorry, we went out of town to visit family this weekend. Just now caught up

After reading thru some of the posts, I think there is something I need to clear up about PRX

Most seem to view the barrel as a beam only. Although it is true that it is a beam, it also is a tube. And being a tube, it creates its own set of harmonics. In particular, we are talking about a closed end tube. Closed end tubes are unique in that they develop odd harmonics only. The physics of the harmonics of a closed end tube are well documented.

The post regarding PRX setting a length that allows for a node too be at the muzzle is spot on

A hypersonic bullet may depart the barrel before a standing wave can develop in the beam aspect, the gas (air) ahead of the bullet is being compressed and moving towards the muzzle. As the gas moves thru the barrel, harmonics will develop. And as the gases escape past the muzzle and pass the bullet you can actually see nodes and antinodes (high speed photography)

Without a whole lot of data or experience to rely on, I am of the belief that the CF rifles will tune best at the higher harmonics since we are dealing with hypersonic velocities

Well, time to pack up momma and go find a place to view the 4th of July fireworks

Have a good nite

Tony


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