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  #1  
Old 08-07-2017, 09:10 PM
PakRat56
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PRX and the Long and Short Slide



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Hi All

Way back when, when I first started working up PRX, the only thing I centered on was the Harrels tuner. In actuality, I believe, and said as much, that if one did the match and cut a tube of material that was kin tot he barrel, to the correct length, you would be in tune. I still think that at a personal level.

The site I initially posted on (this was before I officially gave it a name)did not buy onto the application of "ornaments". But I coudn't help but notice that shooters were having success with these slides, and I decided to see if the same process would apply to these as well.

The long slide fit the process almost perfectly. Instead of using the 9th harmonic like I did for the Harrels, I started running thru the lower harmonics to see which fit best. Turned out the lowest harmonic, the 3rd worked to a tee. The formula remained the same, only the numbers and the End Correction changed

An example I used was for a 24" barrel:

Instead of dividing the barrel by 8 like we do for the 9th harmonic, we divide by 2 (one less than the harmonic we are tuning to)

24 / 2 = 12. This is our quarter wave length

We get our Acoustic length by multiplying the QW length by the harmonic we are using.

12 X 3 = 36

Both the long and short slide have the same inside diameter of 1.5". By multiplying 1.5 times .3 (standard EC factor), we get .45

We now subtract the EC from the Acoustic Length

36 - .45 = 35.55

This is what the overall length needs to be from the end of the chamber to the end of the tuner.

By removing the original barrel length we get the length of the tuner from the muzzle to the end of the tuner

35.55 - 24 = 11.55

The long slide is 9" long, so if we remove this as well, we find out how long the Harrels needs to be

11.55 - 9 = 2.55.

This same formula works extremely well for bloop tubes, and has been successful in international competition and the Olympics. Bloop tubes are kind of near and dear to my heart, since they do not adjust for length.

Now the short slide was a different animal to figure out. No matter what harmonic I tried to work, the numbers just would not fit.

Then an odd thot came to me. Why can't there be a QW node in the tuner as well as at the muzzle. In other words, why cant the tuner be 3 QW long instead of just 1. When I applied that to the short slide, it fell into place like it was made for it.

Lets use the 24 barrel as an example again. Instead this time we are going to use a bit higher harmonic but modify the process. For this, we will use the 11th harmonic.

Since there will be 11 QW in new tube tube, and we know that the tuner will be 3 QW long, we know that there will be 8 QW in the barrel. So we will divide the barrel by 8 to get the length of the QW.

24 / 8 = 3

Now the overall barrel and tuner needs to be 11 X the QW length

11 X 3 = 33. This is the overall acoustic length

Like the long slide, we need to subtract the EC of .45

33 - .45 = 32.55 Chamber to the end of tuner

When we remove the original barrel length of 24"

32.55 - 24 = 8.55

the short slide is 6"

8.55 - 6 = 2.55 Harrels tuner length

This tune works extremely well for the bloop tubes too

This has been kind of a down and dirty explanation of PRX and the slides.

I will answer any questions to the best of my ability

Tony Purdy
The Purdy Prescription (PRX)
Does your rifle shoot "Purdy Good?"
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  #2  
Old 08-07-2017, 09:54 PM
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Tony, were the folks that were having success using slides and noodles using the lite or heavy washer?

marty
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Old 08-07-2017, 10:15 PM
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I tried the prx on a suhl I had. Didn't work. Tried it on a factory anschutz and it still did not seem to work. But the anschutz seems to shoot as good without a tuner. I now have a 2500x with a .900 strait. I tuned it with the hopwell method over and over because I could not tell wher the best spot was. Finally ended up with a tune and I think it is on the prx number. But it still did not group like I wanted so I bought a short slide. Screwed it on and still not great. I slid the weight all the way out to the end and it seems to start tightening up. Slid the weight back in little increments and it seemed to group about 3/4 inch from the end. That is where it is now. Now I am a sure this setup is probably not where your number add up. So how is it that it shoots the best that I have tried so far?

I think my barrel is 24 5/8 inches. I think my tuner is on 423. I may not be in the ball park. So somebody give me the number of the just the tuner. And then the number with th short slide so I can try them and compare if I am off

Last edited by Highdollar; 08-07-2017 at 10:22 PM.
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Old 08-08-2017, 06:46 AM
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Thanks for the explanation Mr Tony. Your reasoning for the slides and bloop as it relates to PRX brings up one more question. If the PRX is based on the acoustical length of a closed end tube, why can't you just cut the barrel to the PRX length you need to have the rifle tuned to the PRX? Seems like it would be the perfect solution. Thanks again.
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Old 08-08-2017, 01:18 PM
Hoot
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Quote:
Originally Posted by Highdollar View Post
I tried the prx on a suhl I had. Didn't work. Tried it on a factory anschutz and it still did not seem to work. But the anschutz seems to shoot as good without a tuner. I now have a 2500x with a .900 strait. I tuned it with the hopwell method over and over because I could not tell wher the best spot was. Finally ended up with a tune and I think it is on the prx number. But it still did not group like I wanted so I bought a short slide. Screwed it on and still not great. I slid the weight all the way out to the end and it seems to start tightening up. Slid the weight back in little increments and it seemed to group about 3/4 inch from the end. That is where it is now. Now I am a sure this setup is probably not where your number add up. So how is it that it shoots the best that I have tried so far?

I think my barrel is 24 5/8 inches. I think my tuner is on 423. I may not be in the ball park. So somebody give me the number of the just the tuner. And then the number with the short slide so I can try them and compare if I am off
Interestingly, I plugged your dimensions and tuner setting into PRX-II and its just about spot on to what the calculator says it should be. The difference could be attributed to the presence (or not) of the blue washer(s) and or measuring tolerances.

Some folks forget to factor the head space and crown into their overall barrel length when computing PRX. IE: My barrel length was cut to exactly 25.5 measured with a rod down the bore to the closed bolt, but is more like 25.44, when they are factored in. I do not understand the roll that the sliding weight plays mathematically. I know that varying its placement does impact the ultimate tightness of groups however, just not why.

Hoot
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Old 08-08-2017, 08:54 PM
PakRat56
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Quote:
Originally Posted by Highdollar View Post
I tried the prx on a suhl I had. Didn't work. Tried it on a factory anschutz and it still did not seem to work. But the anschutz seems to shoot as good without a tuner. I now have a 2500x with a .900 strait. I tuned it with the hopwell method over and over because I could not tell wher the best spot was. Finally ended up with a tune and I think it is on the prx number. But it still did not group like I wanted so I bought a short slide. Screwed it on and still not great. I slid the weight all the way out to the end and it seems to start tightening up. Slid the weight back in little increments and it seemed to group about 3/4 inch from the end. That is where it is now. Now I am a sure this setup is probably not where your number add up. So how is it that it shoots the best that I have tried so far?

I think my barrel is 24 5/8 inches. I think my tuner is on 423. I may not be in the ball park. So somebody give me the number of the just the tuner. And then the number with th short slide so I can try them and compare if I am off
Good Evening Highdollar.

Tuner alone - 2.891 (sorry, I don't provide tuner number, since I cant be sure the bore fro the barrel is consistent tuner to tuner)

Short Slide - 8.784 muzzle to end of slide.

These numbers are based on a 24.625 barrel length

Let me know how these match up. I look forward to helping if I can

Tony
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Old 08-08-2017, 09:07 PM
PakRat56
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Quote:
Originally Posted by drknite View Post
Thanks for the explanation Mr Tony. Your reasoning for the slides and bloop as it relates to PRX brings up one more question. If the PRX is based on the acoustical length of a closed end tube, why can't you just cut the barrel to the PRX length you need to have the rifle tuned to the PRX? Seems like it would be the perfect solution. Thanks again.
Hi Drknite

Unfortunately, no matter how long or short a barrel is there will always be a node at the breach/bolt end, and an anti-node at the muzzle. That is the nature of a closed end tube. Just as a closed end tube only forms odd numbered harmonics. We want the bullet to exit at a node

Now if we were to counter bore the muzzle, we can cut back to where a node would be at a given harmonic, and all would be right. My little Pac-Nor barrel is counter bored to the 19th harmonic, and is only about an inch deep. I haven't tested it as much as I would like but it shows a lot of promise. I'm thinking about building up a little silhouette rifle with this barrel, or possibly a Charger pistol.

Hope this helps

Tony
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Old 08-08-2017, 09:25 PM
PakRat56
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Quote:
Originally Posted by Hoot View Post
Interestingly, I plugged your dimensions and tuner setting into PRX-II and its just about spot on to what the calculator says it should be. The difference could be attributed to the presence (or not) of the blue washer(s) and or measuring tolerances.

Some folks forget to factor the head space and crown into their overall barrel length when computing PRX. IE: My barrel length was cut to exactly 25.5 measured with a rod down the bore to the closed bolt, but is more like 25.44, when they are factored in. I do not understand the roll that the sliding weight plays mathematically. I know that varying its placement does impact the ultimate tightness of groups however, just not why.

Hoot
Hi Hoot

As with everything else, I don't figure weight as a factor. That does not mean weight does not play a roll. We are talking a vibrating rod as well as it being a tube. Any change in weight placement, be it by moving the tuner, or the ring on the slide, it affect the vibrations. But these vibrations will remain constant providing there are no other changes or movements of the weights. So there is merit to moving weight. Many shooter however don't even use the ring. Some have even ordered the slide without the holes

Early on I said that a tuner needed to made a material that is kin to the barrel, ie metal (although now, I am seeing carbon fiber as a viable material), else-wise we could just tape up a toilet paper tube to the end of the barrel and be tuned.

My original thot before even putting out PRX officially, I view this method as a way a shooter could, at the bench, measure the barrel, use a stubby pencil and piece of paper (or his smart phone), do a little math, and set his tuner, and shoot. That in retrospect was a bit of pie in the sky thinking. But I did tune 7 rifles at Nationals a few years back during rotation, and 6 of them shot better. So it can be done.

Hope this clarifies it some

Tony
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Old 08-08-2017, 09:33 PM
PakRat56
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Quote:
Originally Posted by earnest View Post
Tony, were the folks that were having success using slides and noodles using the lite or heavy washer?

marty
Hi Marty

If you are talking about the Blue washers, they have used both(not together both) and neither. I have never discourage using them. Often just that little extra bit of length is the difference between hitting the harmonic and missing it. Mr Pappas came up with them for a reason, so if a shooter chooses to use them, so be it.

I have never heard back of any detriment either way

Hope this answered your question

Tony
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Old 08-09-2017, 09:42 PM
ArtS
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Tony

Been off for a while but just happened to see this thread.

Quite a while ago, I asked you about the carbon fiber bloops you helped work on. I had been asked to measure the tubes that came from the factory with the 54 and the longer one with the 54-18. I noticed that the long carbon fiber being sold and the long Anschutz factory were close to fitting the formula. The shorter in both types didn't come close. I calced and calced and realized the only way the short ones can work is to have a node in the tube and at the end of the tube. I wrote and asked about this and you said you couldn't respond in detail due to confidentiality, but that you had discovered that many multiple node solutions were possible. If you can now discuss it fully, I would be interested in your comments in two areas. First, multi tube nodes on short barrels. This is an issue on auto benchrest guns due to the tuner generally being longer than the length needed for a tube. The other is the potential use on centerfire BR. Can a multimode solution be used at CF frequencies to make the tuner easier to tune?
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  #11  
Old 08-10-2017, 01:10 AM
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Thanks guys. Never sure if my math is correct. Looks like the tuner is nearly spot on like Hoot said. The short slide I have been just shooting where the tuner is set and doing good. But your numbers say it needs to be screwed in toward the muzzle about 70 thousandths. I will try that next time. I was just amazed it shoots good just where the tuner was set. My slide has no holes. I actually bought it as a long slide and cut it off to turn it in to the short slide

Last edited by Highdollar; 08-10-2017 at 01:13 AM.
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