TargetTalk

PRX2500 wrote:...Pick a harmonic, subtract one, divide the barrel by that number, multiply by the harmonic number, subtract your original barrel length, and you are left with the length of tuner you need to tune the barrel.

Tony,

thank you for the Layman's explanation; eeven my appalling maths can understand that. So a 6in tube on a 24in barrel would hit the 5th Harmonic (5 - 1 = 4, 24 / 4 = 6, 6 x 5 = 30 - 24 = 6). How do you tell which harmonic you want though?

Tim

Good Morning

You are correct Tim.

We have one last thing to do. When we multiply the QW by the harmonic, we get what is called an acoustic length. The acoustic length takes into consideration that the harmonic wave extends past the muzzle a short distance before reflection back down the tube. What we are looking for in is a physical length. We get this by using what is called an End Correction. The EC is based on a percentage of the opening diameter of the tube you are using to tune with. The EC I use is.3. Depending on whose research you use, I have seen the EC be as high as .4.

Lets say your tube has a .75 opening diameter. We would multiply .75 X .3 = .225(I corrected this number from this morning, was I a hurry and left off a 2). We would subtract this number from the tube length to get the actual physical length we need to tune to the selected harmonic.

Selecting the harmonic is just as simple as deciding which harmonic gives you a the length of tube you want.

Got to head to work

Have a good day all

Tony

Next Up, multiple harmonics in a tube.

Remember those old radios in the car back in the good old days. You have to turn the knob to your favorite station? There may be several station available to tune to and they are all assaulting the radio at one time, all the time, but we just tune them out and tune our favorite in.

PRX is kind of the same thing. By tuning to a specific harmonic we can mute the actions of the other harmonics.

You are also correct in that PRX deals only in the odd harmonics found in a tube that is closed on one end and open at the other. I have been able to tune barrels with as low as the 3rd harmonic (this is as low as you can go) and as high as the 21st. Past the 21st harmonic, the nodes and anti-nodes start really piling up and small changes in length tend to create havoc with tuning.

Choosing what harmonic to use is just a matter of figuring out how long a tube you want and then running the numbers to see which one gets closest to your goal. I do have a few tricks that allow me to get hard to reach lengths. I originally started PRX working with the RF Benchrest crowd. The harrels tuner is probably the most common tuner used. For most barrels, the 9th harmonic fits quite well. We also have add on tubes, and I have been successful tuning them as well

When I worked with Guy, he had a couple of specific lengths he wanted, and I was able work it out for him

Without the weight on Guys tuner, consistent ammo of different lot speeds would just print to different spots and all we need to do adjust our point of aim. With the addition of the weight on the SCT, Guy is able to tune the mechanical aspects of the barrel vibration to get PC. Length = tuning tube, Weight = tuning the bar.

Have a good nite all

Tony

Hmm interesting thread this one...

So on a FWB 2700 which has a barrel length of 26.75" (without the FWB barrel sleeve/bloop) would have a 9th Harmonic of 29.66" and a 7th of 30.77" (the end, inner diameter of the sleeve is 0.77")

(figures calculated using a PRX spreadsheet I found on another forum).

But with the factory sleeve fitted, the PL is 30.12"

Therefore based on PRX its actually 0.65" too short or 0.46" too long - which given the kind of adjustment you see on tuners is a fairly big difference from the odd harmonic.

Wonder why they picked that length?

Its actually pretty close to the 8th harmonic PL of 30.14" (which is excluded from PRX as its a single ended tube.....)

Justin

Hi All

Interesting comment Joost. There may not be anything to length FWB other than to increase the sight radius. Having an overall length in mm might shed some light. IE, if they say its 100mm, it may be just a length they decided on, if it measures out to an "odd" number like 137mm, may show some sort of thought process involved. Who knows.

Has FWB always shipped the rifle with the tube installed or is this something new?

Would I be correct in assuming the FWB 2700 belongs to you?

Have a good nite

Tony

30.12in is pretty close to the maximum barrel length of 762mm allowed under the old UIT Standard Rifle rules. I think FWB added the tube in the '90s, so it may be related, or just a coincidence.

Tim,

You may be onto something there, although I used a reversed cleaning rod to measure the distances I could have been more accurate with the measurements.

I made it 765mm and very likely I could be 3mm off. This model is referred to as their Standard rifle.

Interesting that length is pretty much as bad as it could be from a PRX perspective!

I also had a look through my clamped batch test results and theres definitely vertical stringing going on - for all three vendors I've tested.

Tony

Yes its my rifle although second hand - its around 8 years old I think and shipped with the extender, they still do for most of the FWB rimfires.

Looks like a tuner may be in my future...

Thanks for the responses!

Justin

Thanks for everyone's inputs, I didn't expect this thread to get quite so long.

Anyway I've ordered one of Guys tube's today and will look forward to testing it (my £218 grunig tube is now redundant less than a year after buying it!).

I worked on the theory that a lot of top level shooters are using one and they test equipment a lot more than I do, hopefully some of the recent 9.9's might creep into the 10.0!

I'll try and report back once it's fitted and tested.

Bryan

Interestingly I've just received this email reply from Dr Geoffrey Kolbe:

"The 200 gm weight was lucky happenstance - I did not try others or determine the weight through trials. I should state too that the rig on which the tests were done was a 'case study' and nowhere did I say that this represented in any meaningful way a rifle shot from the shoulder. Moreover, subsequent trials that I have done with conventional rifles shot from the shoulder or shot on a free-recoiling rest show that this indeed the case.

In such rifles, exhaustive tests have shown that adding weights to the muzzle of the barrel (and some trials with weights elsewhere on the barrel) usually does very little to change the vibrations. The vibrations can be damped and if the weight is sufficiently high they can be 'stopped' (to use a Calfee expression - he was right there), which can lead to an improvement, but it seems they cannot be altered in such a way to turn negative compensation into positive compensation as was possible with the rig shown in the article. A notable exception was the addition of a foresight extension tube (Starik tube) to the front of a Walther rifle(s) altered the vibrations significantly for the better!

My conclusion (at this point) is that the vibrations most probably come from the fall of the firing pin and are not barrel generated by recoil about the centre of gravity - as they are in the case of centrefire rifles. So for a given rifle, it is difficult to alter the vibrations to achieve positive compensation by adding weights to the barrel.

Obviously, best accuracy will be achieved by achieving positive compensation in a given rifle, but I have not been able to determine a reliable way to achieve this goal as yet."

So the Anschutz and Bleiker split receivers may be affecting the firing pin vibrations traveling down the barrel? If his results are correct, what does it tell us about selecting ammo with the barrelled action out of the stock?

"So for a given rifle, it is difficult to alter the vibrations to achieve positive compensation by adding weights to the barrel". So tuner weights are placebos?

Interesting...

Mark

Hi All

Bryan, would it be possible for you to forward Dr Kolbe my email address. I have some questions I would like to ask.

prx2500@outlook.com

I appreciate your help

Have a good evening

Tony

Tony, Dr Kolbe's email address is on his website, here:

http://www.geoffrey-kolbe.com/

Drop him an email and I'm sure he'll reply.

Bryan

Bryan996 wrote:My conclusion (at this point) is that the vibrations most probably come from the fall of the firing pin and are not barrel generated by recoil about the centre of gravity - as they are in the case of centrefire rifles. So for a given rifle, it is difficult to alter the vibrations to achieve positive compensation by adding weights to the barrel.

Good afternoon Bryan:

I had to read this statement a few times, and I am still not sure I fully digest it. It doesn't make sense to me that the only source of vibration is the firing pin strike. Maybe I am misreading it, or misinterpreting it?

I have access to some fairly sophisticated FEA software here at the office (although not specifically tailored to this application), and have tried to model the movement / material reaction as the detonation takes place the bullet begins to accelerate. Modeling the parts and material properties is not too difficult, but applying the correct forces and their subsequent vectors is (unfortunately). Therein lies the problem and the heart of the question I am afraid.

Vibration IMHO would come from the mechanical movement (firing pin, of course), but also from the detonation itself, combined with the lateral forces from the bullet forced to follow a path that may not be its true centroid - combined with the torsional force as it engages the rifling and ramps up its rotation.

And I do know that adding (or subtracting) significant amounts of weight can bring a rifle (as a system) into or out of tune. I have had several people ask me to barrel and tune rifles with "naked" barrels - with the thought that if the rifle shoots well in that state, adding an extension tube / tuner could only help the situation - which in my experience it has not (yet). Adding or subtracting significant weight (say, more than a few ounces) is a complete crap shoot. Sometimes it does, sometimes not - but it seems completely random and beyond any calculations or theory I am aware of. It does completely change the performance and dynamics of the system though - that I know for sure from very practical experience.

kev

Let me say I am brand new to the board. Tony mentioned this thread on another board, and I read it and would like to make a comment.

My interest is rimfire benchrest. Over the last year, I have built about 15 to 20 benchrest rifles. I talked to Tony at some length about the Purdy RX last year, and think I pretty well understand it. Unfortunately that is not generally true. After talking to him, I decided to add the Purdy RX to the Harrel tuners I was using. They sell screw in bloop tubes for their tuners which allow you to independently tune both the vibration and acoustics of the barrel. ( the weights move in and out, but the bloop extension is fixed and extends past the tuner. My general method is to shoot the barrel without a tuner and select a good lot of ammo. I then add the tuner and tune for the best groups with that ammo. At that point I cut a bloop tube to the proper Purdy length and add it. In every instance but one I have always achieved significantly better accuracy after adding the tuner, and again after adding the tuned bloop tube. In the lone instance, I was playing with a very short and stiff barrel, and the bare barrel groups were so tight I'm not sure I could have seen the difference.

My conclusions after doing this a number of times in one year is that weight tuners do provide positive compensation for a barrel. It is a sensitive adjustment, and only a couple of clicks on a tuner can make a big difference. Purdy tuners also work and have a significant impact on accuracy, although not nearly the effect of compensation tuning. My belief is that by positioning the proper node at the muzzle, you minimize the effect of the muzzle discharge on the bullet, leading to better stabilization. I think best results are obtained by using both of them independent of each other. Many people use a Harrel type tuner and try to adjust it for a Purdy tune, and only by serendipity can you achieve a good tune that way.

the discussion above about acoustic tones is true. Blow a horn and you hear a single primary tone. I build musical instruments and use computer software to help tune such things as guitar tops, banjo rings, violin tops etc. Until you see a trace of these sounds, you don't realize how complex they are. If for instance you tap a guitar top it will emit a recognizable tone, but the trace will show dozens of harmonics and overtones. It is easy to pick out the ones containing the most energy. If I ever get the chance to work on it, I would like to ring a barrel, then select several of the harmonics and try different tubes at each of these.

kevin nevius wrote: I had to read this statement a few times, and I am still not sure I fully digest it. It doesn't make sense to me that the only source of vibration is the firing pin strike. Maybe I am misreading it, or misinterpreting it?

Vibration IMHO would come from the mechanical movement (firing pin, of course), but also from the detonation itself, combined with the lateral forces from the bullet forced to follow a path that may not be its true centroid - combined with the torsional force as it engages the rifling and ramps up its rotation.

And I do know that adding (or subtracting) significant amounts of weight can bring a rifle (as a system) into or out of tune. I have had several people ask me to barrel and tune rifles with "naked" barrels - with the thought that if the rifle shoots well in that state, adding an extension tube / tuner could only help the situation - which in my experience it has not (yet). Adding or subtracting significant weight (say, more than a few ounces) is a complete crap shoot. Sometimes it does, sometimes not - but it seems completely random and beyond any calculations or theory I am aware of. It does completely change the performance and dynamics of the system though - that I know for sure from very practical experience.

kev

Hi Kev

To be honest, I dont get it either. After re-reading his website I'm now questioning why he even did the test the way he did, it obviuosly wont show any correlation with shooting from the shoulder if you clamp around the barrel. Watching the barrel on my anchutz whilst batch testing shows much it vibrates from the round going off and although most will be absorbed in the shoulder not all will be.

I've also seen many benchrest articles where the shooters have added kilos (not grams) to the barrel to improve group size, I had good results at the weekend adding tyre weights to my tube to bring its weight upto the same as Guy uses. It also slowed the hold down a lot, whether it was this that actually gave the results I cant say.

ArtS, thanks for joining and your comments. Can you rember which barrel manufacturer made the short stiff barrel that performed well?

As Tim has already mentioned, this is beginning to get above my pay grade! I guess I'll find out how good they are when my starik tube turns up.

Kevin,
I have different experience than you had.I have tested many rifles in my shooting career, and every one of them shot better with a bloop tube,and a tuner.(I haven't tried reverese tapered barrels).
From my experience,with the right placement of the tuner,and the right length of tube,every rifle will shoot better than it will shoot naked.( I test with a test bench which recoils up and backwards).
Guy.

Guy:

Thanks for the reply, I hope you had a great weekend!

I think I may have confused my position with my previous post....i believe tuners or extension tubes that combine tuners are a huge advantage. Unless you do your own work, having some means to optimize accuracy is invaluable.

You can get naked barrels to shoot well....in some cases as well as a tuned system, but it takes testing and setting the barrel back as you try to approach a "node", or sweet spot. It is extremely labor intensive, and once material is removed it's gone forever. I use the same approach using a tuner / tube....as I mentioned previously, I set the rifle up as I would use it in competition and use the tuner as a mechanism of fine tuning.

The only point I was trying to make (and my apologize for the fork in the road), is that if someone is fortunate enough to have a great shooting rifle (naked), it is not a certainty that the addition of a tuner / extension tube will always improve the performance.

All the best,

Kev

"is that if someone is fortunate enough to have a great shooting rifle (naked), it is not a certainty that the addition of a tuner / extension tube will always improve the performance"

A good shooting 1813 that liked Lapua did not group any tighter at 100 with the addition of a BeeSting tuner; an expensive experiment. It took a few trips to the range to find a setting where the tuner would not hurt performance. It did improve the Eley and RWS groups, but they never matched the Lapua. This was a dedicated scope rifle; perhaps it needed the added weight of the front sight.

Mark

patriot wrote:A good shooting 1813 that liked Lapua did not group any tighter at 100 with the addition of a BeeSting tuner; an expensive experiment. It took a few trips to the range to find a setting where the tuner would not hurt performance. It did improve the Eley and RWS groups, but they never matched the Lapua. This was a dedicated scope rifle; perhaps it needed the added weight of the front sight.

Mark

Mark, I think you may have missed the point of the tuner. It will tighten up vertical groups were there is a speed variation across the ammo. The fact it showed an improvement with Eley and RWS proves it's working. I suspect you got lucky with the Lapua and have a very good batch with very little speed variation across the ammo, hence no benefit. It's still important to choose the right ammo for the barrel.

I'm currently also shooting Lapua Midas+, for the first time, and I'm very impressed by it. I'm interested in going to Eley to take them up on their Eley challange to see how it fairs. I'd also love to get my hands on a few hundred rounds of x-act to see how it performs however the NSRA will only take orders for 5000!

Bryan

Bryan,
I chronograph all my ammo; some lots are beyond help.

I've seen tuners reduce vertical on other rifles; but to Kevin's point not in this one case. Groups of the same ammo in my 1913 improved using the BeeSting and it made the 1813 more brand/lot tolerant. The RWS had decent numbers but shot like .... (terrible). Burn rate? Run out? Bullet Engraving? Performance becomes more apparent past 50 yards, when conditions aren't fouling up the results.

If we could buy or load ammo with a very small ES then would any of this matter? Is Geoff's test saying we are all delusional? If vertical can be increased in any number of ways, then why doesn't it follow that it can be decreased?

I recently tested some X-Act; wish they had had more than one lot. The ES and SD were good, but not as good as three lots of Midas tested this spring. It does shoot tight groups at 100 in the 54.30. Is there is some external ballistics variable in play that can't easily be measured? I've looked at velocity loss at 100 using two chronographs. One test of the Eley EPS bullet vs. the Lapua round nose showed more drag. This would support the notion that it does not perform as well in the wind. But on a second test it wasn't clear. Just a small change in the head/tail wind component would be a factor. How many tests using different rifles would it take to know? Maybe after I retire I'll get access to a 100+ yards indoor space and follow Landy's example improving the measurement equipment. Or maybe I'll spend more time kayaking...

Mark

Is anyone intrigued by this notion of measuring when a bullet goes to sleep?