PRX Formula General info Please

S

steve br

Guest
Hi,
Could someone explain (or post a link to) general information about the PRX tuning method? Also how would one use this formula to tune a rifle with a Harrels tuner?
Thanks
 
Purdy PRX

If you have Excel 2003 or later on your computer I can send you a link to an excel program I developed for the Purdy PRX.
Doug
 
Okay

her is an example for you.

Take your overall barrel length, and divide it by 8(quarter wave value). Take that value and multiply it by 9(the harmonic value you want to tune to). From that value you subtract the EC(end correction value). This will give you the overall lenth that your barrel should be with the tuner on. In other words you would slide your tuner on to your barrel, tighten it down, then screw it in or out until you get the length you calculated.

The divide by 8 and multiply by 9 are the common values used for roughly a 26 inch barrel. If your barrel is longer or shorter you may have to calculate this with a different quarter wave value and different harmonic value. Whatever harmonic value you chose the quarter wave value is one lower. Example: 6th harmonic the quarter wave would be 5.

So my rifle has a 26.375 inch barrel. The end of my Harrels tuner with one weight ring on it has a hole diameter of .935 inches. The calculation is such. The EC is calculated by taking the diameter of the hole of the tuner used, in this case .935 inches X .3=.280

26.375/8=3.296 inches X 9=29.671-.280=29.391 inches. The overall length of my barrel from bolt face to end of Harrel's tuner needs to be 29.391". That is flush to the edge of the tuner.

This is just using the standard Harrel's/Hoehn tuner without a bloop tube.

It seems complicated but once you know how to play with the quarter wave values, harmonic you want to tune to and how to calculate the EC. It comes easy. You can play with the numbers for different length barrels and see which harmonic you would have to use so you have enough adjustment in your tuner.

The little bit I have read it may be best to try and keep the tuner as close to the middle of its adjustment range or maybe out to the extreme end.

Not sure if this helps.

Tony Purdy has a nice explanation of the Formula.

Calvin
 
Hi D

A lot of it depends on what you would to tune with, a Harrels alone (or with weight rings as needed), the short slide, or the long slide. Each will help you tune to certain harmonics. If your willing to experiment with different tube lengths,, any of the harmonics become available

With a harrels out at about 350-400 should tune a barrel that long at the 7th harmonic, but that was just a quick run at the numbers. Knowing the actual length of barrel will change the results.

Let me know how I can help

Tony
 
Tony -

Thanks for the reply !.!.!

Here is my specs and particulars to my scenario:
Caliber..................... 6Dasher
Barrel Length............ 29.880" (crown to bolt-face)
Tuner Brand.............. Ezell "PDT"
Tuner Type.............. 7-oz Center-fire (no weight rings)
Tuner Hole (ID) ........ 0.745"

Appreciate the help/input....
Donovan Moran
 
How much

Movement/ adjustment do you have in the tuner?

When it is mounted on the barrel, at mid point adjustment, what is the length from bolt face to the end of the tuner?

This will help in calculating from this end.

Just using common figures I get

29.880/8=3.735x9=33.615-.223(EC)=33.392"

If you cannot make that length at the mid point of adjustment we need to play with the 1/4 wave and Harmonic values.

Calvin
 
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dmoran65 said:
Tony -

Thanks for the reply !.!.!

Here is my specs and particulars to my scenario:
Caliber..................... 6Dasher
Barrel Length............ 29.880" (crown to bolt-face)
Tuner Brand.............. Ezell "PDT"
Tuner Type.............. 7-oz Center-fire (no weight rings)
Tuner Hole (ID) ........ 0.745"

Appreciate the help/input....
Donovan Moran
Click to expand...
Donovan
The PRX has been great for rimfires, but it doesn't work the same with centerfires. Tony can be much more specific and detailed than I where this is concerned. I spoke with him about this, and from what I gathered, somewhere around...and I mean a big around..the 27th harmonic MIGHT be worth testing on centerfires.
My tuner is not meant to be able to be used with the PRX for centerfires..just rimfires. I'm playing with it, but it seems a big stretch at this point to claim that there is any hard, fast length that will allow the PRX to be applied to centerfire rifles.

The rimfire version is a different story. I'm making them with a clamp on threaded inner portion that I am cutting to the customer's nearest PRX.

Bottom line...If you want to try it, there's certainly no harm, but neither my tuners nor the PRX were intended to be used for tuning by length on centerfires. It works great on rimfires in conjunction with the PRX, though.--Mike Ezell
 
Hi All

PRX and center fires. That is one of the projects I am trying to get more information on. What I am finding, like Mike said, is that CF rifles seem to "operate " at a much higher frequency. When trying to tune at these higher frequencies, the incidence of QW's start to get really close to each other. What that means is that the separate harmonic tunes are spaced much closer together. The 27th may be a bit high, but the 21st is definitely in the running.

One of the things to consider as well, is the pressures at the muzzle. If the tuner is too long it will require that the tuner will need to be threaded on versus clamping so you don't end up shooting it off the end of the barrel. This would be a bad thing:(

One of the designs I am working on is a shorter tuner with a larger inner diameter. The larger ID would allow for the rapid expansion of gases and the shorter tube would keep pressures from rising too high. If I can get a handle on the pressures, the tuner can be clamped in place

I would really like to do a proof of concept, but I don't have a quality CF BR gun.

Another thing to consider may be counter boring. At these higher frequencies the depth of counter bore may only be only be about an inch or so. But this is a permanent mod to a barrel, and I am not to fond of recommending it, due to the possibilities of turning a great barrel into a tomatoe stake.

I would really like to see what we can do to make this work. Any and all help would be greatly appreciated.

Have a great night all

Tony
 
Mike, Tony, and Calvin -

Thanks for the replies and feedback......

Here is where I'm at with this all:

Seen this thread a while back and it sparked my interest, since I was getting one of Mike's tuners (that came today).
I posted my 1st reply before I yet had it (prematurely), but was putting the formula that Calvin provided into a spreadsheet for later use and was curious to the "values" to use for longer barrels.
Then today when I received it, I measured it and put those specs in my 2nd reply above.

Wasn't planning on the PRX before I order the tuner from Mike, just came across this thread after the fact, and sparked the interest.
I was just kind of "running the numbers"... sort to speak ... so far and that is about it.

The rifle is a 1000 Lt-Gun and the barrel that I will thread up for Mike's tuner is showing okay performance, but not stellar. From which I think it is a great candidate for a tuner, to see if I can get more from it or just replace it. Sort to speak, as is I think the barrel can win, but not set the world on fire...
And hoping I can get more from it with the "PDT" tuner concept that Mike has engineered .

I most likely will not get to trying the tuner out until into June some time at the earliest.
But would be willing to try/test a few aspects to the PRX method, if you guys would like when I do (let me know).

On a closing note to Mike: Great workmanship and quality machining... it really looks great !.!.!

Best Regards
Donovan Moran
 
Hi Donovan

If possible, could you provide the length of muzzle to end of tuner at mid range (just like Calvin noted). With that I could see possibley how high a harmonic we could tune to. Being that it will be threaded on, we should be able to "survive" at lower harmonics. Just not sure how successful it may be. Then again, it may just fit the process perfectly and you end up with a dynamite tune.

I appreciate your willingness to experiment.

Drop me an email at prx2500@outlook.com

I spent a few years up in the Grand Forks area back in the late 80's

Thanks

Tony Purdy
the Purdy Prescription (PRX)
 
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