GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on November 06, 2017, 07:34:58 PM
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There have been some recent discussions about the use of Harmonic Barrel Tuners to dial in a PCP for a given pellet and velocity.... Here are some generic charts showing what happens when the pellet arrives at the muzzle of a barrel that is vibrating in the vertical plane.... The first chart shows an example of the angle the muzzle is pointing, relative to the boreline at rest when the trigger is pulled.... I drew the chart so that a pellet travelling at 900 fps would arrive at the muzzle at the point where the barrel is at its highest.... with that being a bit over 2 MOA (it might be more or less, depending on barrel stiffness)....
(http://i378.photobucket.com/albums/oo221/rsterne/Important/Harmonics%201_zpsolvgx2xf.jpg) (http://s378.photobucket.com/user/rsterne/media/Important/Harmonics%201_zpsolvgx2xf.jpg.html)
You would think that would be the worst possible case, because the barrel is the furthest from its position at rest.... but in fact it is pausing at that moment, because is it reversing direction.... In the second chart (a detailed view of the last peak in Chart 1), each dot represents the time the pellet arrives at the muzzle, with a dot every 10 fps from 800 to 1000 fps.... 900 fps is at the peak, because I chose the vibration frequency to make it so.... You will note that there is hardly any difference in the angle the pellet leaves the barrel for quite a large ES either side of 900 fps.... That intuitively might tell you that is the optimum velocity for a barrel vibrating at this frequency.... However, that is incorrect also....
(http://i378.photobucket.com/albums/oo221/rsterne/Important/Harmonics%202_zpsdqf22vul.jpg) (http://s378.photobucket.com/user/rsterne/media/Important/Harmonics%202_zpsdqf22vul.jpg.html)
The next chart shows the POI of the pellet (I chose a BC of 0.036 to get these numbers at the various velocities).... If the gun is sighted in at 100 yards when the velocity is 900 fps, then at 800 fps it hits about 6.5" low, and at 1000 fps about 4.2" high.... At 800 fps, a 20 fps ES changes the POI about 1.5".... At 900 fps, it makes about a 1.0" difference.... and at 1000 fps, that 2% ES changes the POI only about 0.7"....
(http://i378.photobucket.com/albums/oo221/rsterne/Important/Harmonics%203_zpsdraogpnf.jpg) (http://s378.photobucket.com/user/rsterne/media/Important/Harmonics%203_zpsdraogpnf.jpg.html)
When you combine the flatter trajectory of faster shots with the angle of the muzzle at the instant the pellet leaves the barrel.... you get an interesting result, as shown below (gun again sighted in at 900 fps at 100 yds.).... The slower pellets (eg. @ 800 fps), arriving after the peak of the barrel vibration, shoot lower on the target.... but the faster (1000 fps) pellets, arriving when the barrel is still travelling upwards, ALSO shoot lower on the target.... This increases the vertical dispersion for pellets travelling slower, and decreases it for pellets travelling faster.... Now, for that same 20 fps ES, pellets at 800 fps have vertical stringing of 2.1" (instead of 1.5").... those at 900 fps are still about 1.0".... but pellets at 1000 fps, with that same 2% ES, only have a vertical stringing of less than 0.3" (instead of 0.7")....
(http://i378.photobucket.com/albums/oo221/rsterne/Important/Harmonics%204_zpsunns79pa.jpg) (http://s378.photobucket.com/user/rsterne/media/Important/Harmonics%204_zpsunns79pa.jpg.html)
It is important that you do not interpret this to mean that faster is always better.... The concept I want you to understand is that by using a movable weight (or changing a weight) attached to the muzzle, you can change the frequency at which the barrel vibrates.... If you want to shoot at 900 fps, then you want the barrel to be on the upswing, but nearing the top of it's movement, when the slowest pellet arrives at the muzzle.... This adjustment is arrived at by trial and error, but can reduce the vertical dispersion of your groups once you find it....
Think about it this way.... The slowest pellet in your shot string arrives at the muzzle when the barrel is nearing the highest point on the swing, but still on the way up.... It is launched on a higher angle, but going slower, drops more.... Conversely, the fastest pellet arrives earlier in the vibration cycle, when the barrel isn't pointing quite as high, but going faster, drops less.... The combination of these factors can reduce the vertical stringing of the shots when they impact a target downrange.... I hope this explanation helps you understand the merits of tuning the harmonics of your barrel for the velocity you are using....
Bob
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Hey Bob,
Regulating ones gun between 900 fps/950 fps seems to be the answer ;).
- The barrel harmonics will consistently produce the same angle.
- Any ES will only marginally effect vertical stringing.
- Pellet/Air friction will not a big issue.......> yet.
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I can't argue with most of that.... but at long range, harmonics can make a big difference in POI with just a small ES.... depending on which side of a swing you are.... In addition, although I didn't address it.... barrels don't necessarily just vibrate in the vertical plane (particularly if you didn't index it).... I had a .222 Remington that I handloaded for, and the barrel oscillated in an ellipse measuring about 2" by 1", angled from 1 o-clock to 7 o-clock.... When working up handloads, the groups tightened up at both ends of the swing, and spread out in the middle.... That gave me two velocities where the gun was most accurate.... one right at maximum pressure, and the other about 300 fps less, which was the tighter of the two.... Had I installed and used a harmonic tuner, I probably could have tuned for the tightest group at something in between on velocity.... which would have made the brass last a lot longer, but still given me near maximum power.... Airguns are the same, you may find that your regulated gun groups best at 880 and 980 fps, and isn't that great in between, where you want it to be.... Using a tuner may allow you to move the sweet spot so that the tightest groups occur at 950 fps, just at the top of your desired velocity range....
Bob
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Do you think a tuner will work over a shroud? I kind of think it needs to be on the actual barrel. Which may or not be a problem for some guns.
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Well, you see, that is one of the advantages to living in Canada.... We are only allowed LOUD airguns up here, so we don't need to worry about that.... ::)
Seriously I have no idea.... I'm guessing that if the barrel stops short of the end of the shroud, if it is rigidly attached at the muzzle then a tuner mounted to the front of the shroud.... or possibly one of the sliding weight types.... MAY have some effect.... but I really don't know.... Using a tensioning system to vary the vibration frequency may be more effective.... but a bit harder to retrofit.... The Slayer uses this system, and I think the Flex does as well, they both use tensioned barrels and have shrouds....
Bob
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I drew the chart so that a pellet travelling at 900 fps would arrive at the muzzle at the point where the barrel is at its highest.... with that being a bit over 2 MOA (it might be more or less, depending on barrel stiffness)....
Bob
Bob Thanks a looooooot that you have created post about barrel harmonics.
At yours generic charts you choose such another barrel which is the worst just for 900 FPS ?
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How about the bullets/pellets? Different weight should produce different barrel harmonics as would most likely different valve openings and operating pressures. Also, what would be the effect of a barrel band and it's location in the airtube?
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As usual another Robert Stern instituted migraine but interesting reading none the less. Now for my question would a springer benefit from a CF sleeve glued to it or only PCP rifles ? ie Diana D34
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Any change to the shot cycle MAY affect the barrel harmonics.... I would expect there to be a difference between the start of a shot string and the end, in an unregulated PCP, because at the beginning you have more pressure and less dwell.... while at the end you have less pressure and more dwell.... because of the self-regulating properties of our strike-open valves.... When you change bullet weight, you change the velocity, and hence the time it takes for the bullet to get to the muzzle, so that will for sure make a difference in the optimum tuner adjustment.... This is one thing that is different between airguns and rimfires.... You need to think of a tuner as the last step in a process, to wring the very best accuracy out of a proven combination of pellet and barrel....It should allow you to adjust the barrel vibrations to a certain velocity.... rather than what we do now, which is adjust the velocity to the barrel vibrations....
A Springer may benefit from a stiffer barrel, or from a muzzle weight tuner.... but it won't get rid of the rough cycle (reverse recoil) caused by the piston coming to an abrupt halt at the front of the compression chamber.... I tried a few different weights on my D-34, and found that only a small weight change was needed to tighten up the groups a bit when shooting from a roller rest on the bench.... so I think it can benefit even a Springer....
I tried a series of 6 muzzle weights on a Pumper, and also on a CO2 carbine.... The weights varied only 1/4 oz. each from about 1 oz. to 2.5 oz....
(http://i378.photobucket.com/albums/oo221/rsterne/177%20PCP%20HFT/MuzzleWeights.jpg) (http://s378.photobucket.com/user/rsterne/media/177%20PCP%20HFT/MuzzleWeights.jpg.html)
It was pretty cool to watch the groups shrink and then open up again as you went through the series of weight to find the best one.... even with such coarse adjusting steps....
Bob
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Thanks on the D34 information Bob. Where can I find some of the weights or do you fabricate them yourself, also is the weight installed in place of the front sight ??
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Wow! Great post on a always GREAT forum! love the technical stuff especially when it's said in such a way that Dummy like me can comprehend it!! I have always wondered about barrel harmonics in an air rifle ! I don't claim to know a thing about harmonics but it would seem to me that if an airgun barrel was made like a heavy rim fire bull barrel and not the size of a pencil like so many seem to be that it would have to all but eliminate harmonics would it not? I know our guns are heavy already and that probably plays into the design but I'm just throwing this out there for discussion.
I have found out the hard way(the only way for me I think) that even a slight bit of pressure on the barrel shroud of my Air Arms rifle will change the P.O.I. a great deal so this is very interesting to me!
Keep the great info coming guys!! Jeff L. McMurray Pa.
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Any change to the shot cycle MAY affect the barrel harmonics.... I would expect there to be a difference between the start of a shot string and the end, in an unregulated PCP, because at the beginning you have more pressure and less dwell.... while at the end you have less pressure and more dwell.... because of the self-regulating properties of our strike-open valves.... When you change bullet weight, you change the velocity, and hence the time it takes for the bullet to get to the muzzle, so that will for sure make a difference in the optimum tuner adjustment.... This is one thing that is different between airguns and rimfires.... You need to think of a tuner as the last step in a process, to wring the very best accuracy out of a proven combination of pellet and barrel....It should allow you to adjust the barrel vibrations to a certain velocity.... rather than what we do now, which is adjust the velocity to the barrel vibrations....
A Springer may benefit from a stiffer barrel, or from a muzzle weight tuner.... but it won't get rid of the rough cycle (reverse recoil) caused by the piston coming to an abrupt halt at the front of the compression chamber.... I tried a few different weights on my D-34, and found that only a small weight change was needed to tighten up the groups a bit when shooting from a roller rest on the bench.... so I think it can benefit even a Springer....
I tried a series of 6 muzzle weights on a Pumper, and also on a CO2 carbine.... The weights varied only 1/4 oz. each from about 1 oz. to 2.5 oz....
(http://i378.photobucket.com/albums/oo221/rsterne/177%20PCP%20HFT/MuzzleWeights.jpg) (http://s378.photobucket.com/user/rsterne/media/177%20PCP%20HFT/MuzzleWeights.jpg.html)
It was pretty cool to watch the groups shrink and then open up again as you went through the series of weight to find the best one.... even with such coarse adjusting steps....
Bob
Bob,
Those muzzle weights are made of aluminum aren't they?
If yes, I'm gonna make some and give 'em a whirl.
Kirk
Also, not to sound Naive, but are there after-market barrel tension/tuners sold for PCP's or do you have to make your own. I would love to see some plans :D.
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Yep, aluminum weights.... The barrel fits almost all the way through, and the other end is bored large and countersunk to avoid it having ANY affect on the muzzle blast.... Here my D34 with a larger one installed in place of the front sight.... makes a nice cocking handle too....
(http://i378.photobucket.com/albums/oo221/rsterne/Parts%20for%20Sale/Diana%20D-34%20Left_zpsrlqhcngw.jpg) (http://s378.photobucket.com/user/rsterne/media/Parts%20for%20Sale/Diana%20D-34%20Left_zpsrlqhcngw.jpg.html)
Bull barrels of the length we use would be VERY heavy.... Great if you are only shooting from a bench, IMO the best way to go.... For a hunter, I would sooner sleeve the typical thin barrels we have with a thick Carbon Fibre sleeve....
(http://i378.photobucket.com/albums/oo221/rsterne/6mm%20Sporter/14mm%20OD%20Barrels_zpsgulggcha.jpg) (http://s378.photobucket.com/user/rsterne/media/6mm%20Sporter/14mm%20OD%20Barrels_zpsgulggcha.jpg.html)
On these barrels I will leave a threaded stub at the muzzle, and install an adjustable tuner....
(http://i378.photobucket.com/albums/oo221/rsterne/Parts%20for%20Sale/Barrel%20Tuner_zpsizerxqag.jpg) (http://s378.photobucket.com/user/rsterne/media/Parts%20for%20Sale/Barrel%20Tuner_zpsizerxqag.jpg.html)
Bob
ADDENDUM.... Here are photos of the finished Harmonic Tuners I made for my 6 mm and .257 mentioned in this post....
(http://i378.photobucket.com/albums/oo221/rsterne/6mm%20Sporter/Harmonic%20Tuners%20With%20Scale_zpsenzsxvtc.jpg) (http://s378.photobucket.com/user/rsterne/media/6mm%20Sporter/Harmonic%20Tuners%20With%20Scale_zpsenzsxvtc.jpg.html)
(http://i378.photobucket.com/albums/oo221/rsterne/6mm%20Sporter/Completed%20257_zpsbzvp2ae6.jpg) (http://s378.photobucket.com/user/rsterne/media/6mm%20Sporter/Completed%20257_zpsbzvp2ae6.jpg.html)
Steel sleeve about 2.5 oz, brass weight just over 5 oz.... No results as yet....
RBS
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I was about to bring the carbon sleeve in the picture.. LOL The sleeve can change the harmonics in a good way and giving stability to the platform..
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Those 20 mm CF sleeves will quadruple the stiffness compared to the bare 14 mm TJ's liner.... They will be set into the receiver at least 2 diameters to insure rigidity at that point.... Bare liner weighs 1 lb. 10 oz.... sleeve is 6 oz. for a total of 2 lbs.... If the barrel was steel with a 20mm diameter it would weigh 3 lbs. 10 oz.... Just 6 oz. of CF does the same thing as 2 lbs. of steel would....
Bob
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Bob, looking at your plan and wondering if there is a soft ring or something in the inner area were it touches the barrel and to adjust it you tighten the screw or am I missing something? I do that often, sorry!! also what would you think of putting a carbon tube on the barrel of a PCP as long as it would fit inside of the shroud or I guess you could eliminate the shroud and install a moderator some how on the end of the barrel? Whats you thought! I created a problem this summer for my self this summer at the Pyramyd Cup thinking I would make a quick little"Windacator" like I saw many of the better shooters using on the practice range on practice day ( Friday) so I went and got a few plastic wire ties at the local dollar store and put one through the gap between the air tube and the barrel, it was a little tight going through but not terribly so, Added a piece of mono filament and a snap swivel with a feather tied to the end and WALA!! I was good to go! Long story short Saturday Am at the warm up range my P.O.I. had changed drastically!! I was shooting several yards high, and contemplating changing my settings and then I thought I wonder if could have put enough pressure on the barrel shroud to change the P.O.I so I cut off the plastic tie and sure enough the marks were right on again!! Just goes to show how easy barrel movement can change your P.O.I!!!! Sorry for the long winded Posting but this has been a concern of mine sense that day! This thread is great Keep it going Please!! J.L. McMurray Pa.
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The inner black part (steel or aluminum) is threaded onto the muzzle, and in the case of my CF sleeved barrel, it will tighten against the end of the CF sleeve.... The end of the barrel will be turned down to 1/2" OD and threaded 1/2"-20, to match the inside of the black sleeve.... Once installed, you don't want it to move.... The OD of the sleeve will be threaded 15/16"-28, as will the inside of the brass weight.... The brass weight is split on one side, and notched on the other, so that it can be snugged up on the threads by tightening the 6-32 SHCS clamping screw to prevent it from turning.... Alternately, you could use a setscrew with a nylon end in the brass weight to act as a brake....
The reason I chose 15/16"-28 threads is that I have a lot of my guns equipped with Hatsan Air Strippers, which are just over 15/16" OD (24 mm, IIRC), so they could be threaded for the brass weight to allow me to have a Harmonic Tuner on them as well (without buying another tap and die).... If you wanted to build a smaller unit, you could also use the same threads as a Disco air tube (13/16"-28) and a smaller diameter, 1.25" OD brass weight.... I like 28 TPI for adjusting the weight, because each 10 deg. rotation is 0.001".... Index marks, as shown in the drawing, allow repeatability.... If you have marks every 30 deg. (12 marks, like a clock face).... you can count full turns as "hours" and the index marks as "minutes" and record your settings as 3:45 or 7:20 etc.etc....
The pressure of your plastic cable tie probably raised the muzzle slightly, but not enough to show up as yards of POI difference (unless you were at extreme range).... Most likely it also changed the barrel harmonics and the pellet was reaching the muzzle on an upswing....
Bob
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Thanks Bob,
The muzzle brakes on the break barrels should be easy, just need to learn how to anodize aluminum or spray them with high temp black paint.
I was going to respond earlier, because I couldn't see how the PCP tuner could work if the black 1/2"-20 piece wasn't threaded on the OD. I must have missed that it was. Your last response clears up everything.
Pretty Cool................ 8)
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Bob, Thanks for the explanation! I get it know!! very Ingenious! I assume that's the way a rim fire tuner works also, as always you say things in a way that even I can comprehend!! When I said yards I was referring to my sight settings being off by several yards not that I was shooting several yards high above the target!! sorry for not wording it correctly! J.L McMurray Pa.
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bob, to check if tuner is in the right place or setting in a given pellet velocity, all we have to look for is the highest poi? then back it off alittle maybe?
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It's even easier than that.... Simply turn it a turn at a time and watch the group size.... When you find a spot that has the smallest group, try a turn either side of that, moving it about 30 deg. at a time.... When you find the best spot, try even smaller adjustments to see if you can refine it further.... Don't worry about the POI, just look for the smallest groups.... When done, resight it....
If you change the pellet or velocity, double check it.... I'm guessing it will be within a turn either way, unless you make a huge change in ammo.... but on the other hand I haven't tried it yet.... Chris Alls, who won the 2015 EBR with my .308 cal 200 gr. BBT "Whiteout" told me that adjustments of just a few thou might double the group size, or cut it in half.... He was astounded how small the final adjustments were....
Bob
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thanks bob, i was thinking of look for the highest poi first then do the group shots for fine tuning?
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Should work.... In theory you would want to move the weight further from the muzzle to slow the vibration and move the pellet exit onto the upswing....
Bob
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As usual another Robert Stern instituted migraine but interesting reading none the less.
;D ;D ;D ROTFLMAO I thought it was just me. If I'm luck I understand about a third of it but I'm always fascinated and want to know more
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Now I am confused, are these vibrations, at least those worth mentioning, always vertical? I thought they are in all directions, randomly.
With my 24" 2400kt/Disco(?) barrel (ninja regulated bottle gun) I cut shims from a beer can for the barrel band to put a little upward tension at the end of the barrel... The thought is that since the tube pressure stays constant, it's dimensions would stay constant, and I would place the band all the way at the end to dampen vibrations.
At 50 yards, the best group I have achieved with this gun so far is .35" CTC, groups under .5" are reasonably common, and I am disappointed with anything approaching .75" ...
I never did try the "trombone" trick, looking for optimal band placement. Did I just luck out with my band placement, or did the tension dampen the harmonics as I had intended? I have this shimmed band all the way at the end of the barrel:
(https://lh3.googleusercontent.com/0Lde15knk2HvxDLkR_YGiqe4Hk4wuXDDG6aqK3SqOxmLbkR5aHRh3hKBBK2oKUJyXcJe0S7kU9KnncexWdiW8i-1MZz1IUGQ2a3lDZAI-duayg60O8J-bnI8KDhe8LoedIw3tj9YFqgRuKxV3izU_Y4HW1ZA0G9lHa4Wm2wwYClEjHY8tAllNCV4yrk2Bav-DPFFmyCEBQD1Rh9AmhLpvXAzvJnz_AfqEycVlBk4HETsAInDoD6mK6ZhoqY557Lq5t54oH5XMvBE8h93pRt0xwZKv1-plhdsoydIEqLkGz4PMANV5ZOGsNakmbjUOK5VV0_Hyztx9zouGKSLlj0qFEs9zfxQ3gjzQKTsM59y9UVL7oNXxEj84vQEOcyd7NFARlUwmk7sKqXrvTyuiflavYQNKDdcbuFz3xQ4UPg2Ai91ojs4SN-j6EK3mlok0rY5LK1S_lEyvpm5VmhTvtrcVktSeFZrUU8LHwWnK2N5vVBZul_XaHcCeD0TDUAZYwtAtb1z3q70ltbXXCo0FHG4tJ4vIad_Q04mfh6p-SS2sf55nFbf-cn5o2J2Vw9WrgqQExLePHxDHQGvXM1hg0w5UHgC39dNjWzbAexNo1KvcGk=w1076-h256-no)
I hope you can see this image. I am using google photos. It is a little clunky, but I hate that photobucket has changed their policies...
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Applying powder burner concepts - is anyone here familiar with the work done with rimfires and tuning to a specific harmonic (barrel length)? On a rimfire board I frequent, there was a good explanation and a spreadsheet that would calculate the various lengths to hit the nearest harmonic, greatly reducing the amount of trial and error for finding the optimal tuner setting. I believe it was called the Purdy Prescription and was written by a guy named Purdy. I will see if I can dig it up. Based on my understanding of this thread, I think we are talking about the same concepts and it would be applicable.
Added on edit: I found the original doc and spreadsheet and have attached the doc. Doesn't look like we can attach spreadsheets. The spreadsheet basically does the calculations outlined in the doc for you. I haven't looked it up in a while to see if there has been any refinement. As it is, it worked really well for me with a 10/22 clone.
Thanks,
Joe
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We have talking about barrel stiffness and weight at the end but WHAT ABOUT A STRETCHER??? I cant remember where I have read on it but the benefits were good.. Having a good anchors point at the base and a point to stretch the barrel end with adjustable screws for rigidity seems a good solution for adding stability..
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Barrels tend to oscillate mostly in one plane.... The only way to make sure your barrel oscillates in the vertical plane is to test it and index it.... Putting a weight on the end of a free floating barrel will bend it downwards somewhat due to gravity, and accentuate the vertical movement.... Even if the barrel vibrates sideways, you should still be able to find an adjustment that improves the group size....
Yes, tensioning the barrel helps tame harmonics because it increases the frequency and decreases the amplitude.... Having the outer tube / shroud mounted rigidly to the receiver.... or at least having it fit on a snug shoulder, will help as well, because then the barrel acts like a larger OD where it meets the receiver....
Bob
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Hm. So it seems that the only way to know for sure which way your barrel oscillates would be to test at an indoor range... Outdoors introduces wind that could be nice an still at your bench yet moving faster or even a different direction at your target. At least that is the way it works for me, living at the top of a hill! No wind at all seems to only occur at 3am every other third wednesday after a lunar eclipse for me.
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Yes, on my .25 Synrod, I do screw my shroud down very tight against the receiver. I use an O-ring between them to offset any part that is not perfectly square on the end of the shroud. I also use a Hill barrel band that holds the front of the shroud fairly steady, but does allow the smallest of movement too. It is only tighten down after the shroud is in place. I have the baffle spring compressed tightly as well. All of this seems to work better than the loose fit OEM design. I can clean the barrel with a pull through without POI shift and groups seem to hold steady day to day with these changes. I would definitely like to add a CF sleeve to make the barrel even more rigid.
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Yes, on my .25 Synrod, I do screw my shroud down very tight against the receiver. I use an O-ring between them to offset any part that is not perfectly square on the end of the shroud. I also use a Hill barrel band that holds the front of the shroud fairly steady, but does allow the smallest of movement too. It is only tighten down after the shroud is in place. I have the baffle spring compressed tightly as well. All of this seems to work better than the loose fit OEM design. I can clean the barrel with a pull through without POI shift and groups seem to hold steady day to day with these changes. I would definitely like to add a CF sleeve to make the barrel even more rigid.
I think I have read of the tensioner on a tread about the Edgun R3.... The significance of tensioning on the barrel is what Bob mention earlier.. The harmonics are higher with less AMPLITUDE... Resulting in a lesser change of POI..
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It's even easier than that.... Simply turn it a turn at a time and watch the group size.... When you find a spot that has the smallest group, try a turn either side of that, moving it about 30 deg. at a time.... When you find the best spot, try even smaller adjustments to see if you can refine it further.... Don't worry about the POI, just look for the smallest groups.... When done, resight it....
If you change the pellet or velocity, double check it.... I'm guessing it will be within a turn either way, unless you make a huge change in ammo.... but on the other hand I haven't tried it yet.... Chris Alls, who won the 2015 EBR with my .308 cal 200 gr. BBT "Whiteout" told me that adjustments of just a few thou might double the group size, or cut it in half.... He was astounded how small the final adjustments were....
Bob
Bob, I know we are both in Canada and suppressors are a No-no! But I have powder burners in the states with NFA titled suppressors. Question is if you were to change the position on the baffles with different spacers in the expansion chamber side, do you think it would affect the POI? One of my Sleeved Rem 700's in .308 Win has dropped off in accuracy. The throat is still good and the only thing I have changed is the Suppressor guts with identical parts.
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Yes, it could change the POI, and the group size as well, IMO.... Even just moving the weight of one baffle could change the harmonics.... Get yourself a slide-on "Limbsaver" and try that....
Bob
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Barrels tend to oscillate mostly in one plane.... The only way to make sure your barrel oscillates in the vertical plane is to test it and index it.... Putting a weight on the end of a free floating barrel will bend it downwards somewhat due to gravity, and accentuate the vertical movement.... Even if the barrel vibrates sideways, you should still be able to find an adjustment that improves the group size....
Yes, tensioning the barrel helps tame harmonics because it increases the frequency and decreases the amplitude.... Having the outer tube / shroud mounted rigidly to the receiver.... or at least having it fit on a snug shoulder, will help as well, because then the barrel acts like a larger OD where it meets the receiver....
Bob
Barrel tensioner, neat idea. Basically the same thing happens as tightening up a guitar string I imagine.
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Exactly.... and you can tune so that the muzzle is at the top of the upswing and pausing briefly as it reverses direction.... This tightens the vertical POI downrange....
Bob
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Bob
Would this not along with Chris Longs bbl nodes for tuning and getting the best accuracy possible,
and clocking the bbl etc?
Tia,
Don
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Don, all of those methods are part of the puzzle....
Bob
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Any ideas about what could happen with a (magnum) springer accuracy when you bend the barrel due ‘barrel droop’?
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No idea.... bending the barrel upwards to eliminate droop should not change the frequency of vibration....
Bob
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I’m assuming wrapping the barrel in carbon fiber or some type of fiber reinforced plastic acts as a vibration dampener?
Also, I wonder if some people reporting better groups after getting an heirloom .22lr resleeved is partly due to harmonics dampening of the plastic between the 2 tubes?
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No idea.... bending the barrel upwards to eliminate droop should not change the frequency of vibration....
Bob
Thank you very much! Exactly what I did want to know :D
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Interesting thread. I don't get a lot of time on the forums so I am just coming to this party late. Great info, thanks for sharing it with us. Here are some of my experiences with PCPs.
Harmonics are affected by barrel weights "tuners". In testing one .177 FT competition gun I was able to reduce group size by about 50%. Same gun was also able to go from horizontal stringing to vertical stringing with only 3" of tuner movement along the axis of the bore. LDCs attached to the muzzle can fall into the category of tuners. For a competition FT gun I tune the LDC to that gun.
CF sleeves almost always improved groups.
I index barrels on competition guns because it does have an effect.
Tensioning barrels can improve accuracy, but can be tricky to get consistent POI through temperature changes such as a FT match that starts at 40*F then warms up 20*.
Big bore(like 4 shot/fill 300FPE .45) valve dwell/recoil changes noticeably in those 4 shots and like a springer takes some technique to master. Vertical POI can be affected by that irrespective to velocity change.
Harmonics is interesting. I had an Evanix Blizzard .177 I totally tricked out for FT. The gun shot very well, but being an experimental gun I could not leave well enough alone. I bored out the receiver to go from a .437" bbl tenion to .5" tenion and the gun totally lost it's mojo. I spent countless hours on this project over a years time re-machining breech, crown, indexing, tuning, pellets, and tested three different barrels including bbls that were proven accurate on other guns. I finally gave up thinking I had ruined the receiver suspecting harmonics? I also bought a RAW TM1000 .177 and it shot dismal, 1" at 55 yds. I spent a lot of time on it and got it down to 1/2" groups, but not the precision I was desiring. Both guns had 16mm LW barrels and side lever receivers of similar shape and size. I swapped the barrels on these two guns (required machining breeches) and bingo both guns could shoot 1/4" groups at 55 yds in no wind.
Will Piatt
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Thanks Will
It would make sense that tensioning a barrel would change with temperature, as if any of components being pushed or pulled are different material (Steel, Aluminum) they would expand and contract a different amount with temp changes.
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Exactly why I use a Belleville stack at the muzzle to minimize tension change with temperature when tensioning a barrel....
(http://i378.photobucket.com/albums/oo221/rsterne/257%20Benchrest%20PCP/Barrel%20Tensioner_zpsetigcsea.jpg) (http://s378.photobucket.com/user/rsterne/media/257%20Benchrest%20PCP/Barrel%20Tensioner_zpsetigcsea.jpg.html)
Bob
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Interesting thread. I don't get a lot of time on the forums so I am just coming to this party late. Great info, thanks for sharing it with us. Here are some of my experiences with PCPs.
Harmonics are affected by barrel weights "tuners". In testing one .177 FT competition gun I was able to reduce group size by about 50%. Same gun was also able to go from horizontal stringing to vertical stringing with only 3" of tuner movement along the axis of the bore. LDCs attached to the muzzle can fall into the category of tuners. For a competition FT gun I tune the LDC to that gun.
CF sleeves almost always improved groups.
I index barrels on competition guns because it does have an effect.
Tensioning barrels can improve accuracy, but can be tricky to get consistent POI through temperature changes such as a FT match that starts at 40*F then warms up 20*.
Big bore(like 4 shot/fill 300FPE .45) valve dwell/recoil changes noticeably in those 4 shots and like a springer takes some technique to master. Vertical POI can be affected by that irrespective to velocity change.
Harmonics is interesting. I had an Evanix Blizzard .177 I totally tricked out for FT. The gun shot very well, but being an experimental gun I could not leave well enough alone. I bored out the receiver to go from a .437" bbl tenion to .5" tenion and the gun totally lost it's mojo. I spent countless hours on this project over a years time re-machining breech, crown, indexing, tuning, pellets, and tested three different barrels including bbls that were proven accurate on other guns. I finally gave up thinking I had ruined the receiver suspecting harmonics? I also bought a RAW TM1000 .177 and it shot dismal, 1" at 55 yds. I spent a lot of time on it and got it down to 1/2" groups, but not the precision I was desiring. Both guns had 16mm LW barrels and side lever receivers of similar shape and size. I swapped the barrels on these two guns (required machining breeches) and bingo both guns could shoot 1/4" groups at 55 yds in no wind.
Will Piatt
Thank you Will, Sam and Bob for Enlightening me with this Post. I found out about barrel tensioning and temperature change all quite by accident. I have a 22cal Benjamin Trail NP XL 1100 rifle with a factory barrel shroud. The gun would shoot well but I would notice very slight POI changes from day to day. I was puzzled by this phenomenon for awhile then decided to remove the shroud. Bingo..the POI shifts went away. The trade off was a somewhat louder shot report. I live in the boonies, with few neighbors stretched out along my road, so that did not matter. I suspected that all of this had something to do with barrel harmonics and temperature fluctuations but did not KNOW that to be a Fact. You all have now Confirmed my Suspicions. My gun is by no means a target rifle and I do not shoot it like one. When target shooting I do not use any type of shooting bench or bags. I either it shoot off-hand or I brace my fore-stock hand on a door frame and rest the gun in the ve between my thumb and hand. I am more of a small game hunter and Not much of a Target shooter. I use this gun for small game hunting. While hunting I try to rest the rifle on a tree branch or brace my fore-stock hand on a tree trunk and shoot like I shoot from my garage back door. Sometimes I shoot off-hand while hunting but a rested shot is most preferred. I noticed the POI shifts at 40 yards and beyond. Big temperature swings from day to day and from night to day are normal here in Minnesota. Last night it was -9 F and today it is supposed to warm up to + 28 F. During Winter months, it can be +35 F one day and -11F the next. During summer months, it can be +70 F one day and + 45 F the next. Spring and Fall months are the most variable in temperature. This changing temp occurs all year round and is caused by shifts in the jet stream above us. When the jet stream is south of us it is colder and when the jet stream is north of us it is warmer. If the jet stream is right above us then the temperature can shift around a lot during any single day or night. Thank you folks for this post and confirming what I had previously suspected but did not know for sure. My Best Wishes to You All - Tom
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Since this is about harmonics, does it help any if you know the natural frequency of the barrel? If it would be helpful, there are several apps you can download to a smart phone that will tell you the frequency of a barrel that is hung from a string. G Strings and Guitar Tuner are two of them, but there will multiple frequencies that show up in addition to the natural.
Also, I was fiddling with some wind chimes and ran across a website, www.leehite.org (http://www.leehite.org) that has a lot of reference material buried in the site. In his Wind Chime handbook (a pdf on his site) are links to spreadsheets that will predict the frequency of tubes, and you are able to enter material, ID, OD, and length.
With big, heavy, bench rest barrels, I know that the vibration amplitude is reduced to almost nothing, but what about the frequency. And in that situation (bench rest barrels), which becomes more important, amplitude or frequency?
Interesting discussion.
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Exactly why I use a Belleville stack at the muzzle to minimize tension change with temperature when tensioning a barrel....
(http://i378.photobucket.com/albums/oo221/rsterne/257%20Benchrest%20PCP/Barrel%20Tensioner_zpsetigcsea.jpg) (http://s378.photobucket.com/user/rsterne/media/257%20Benchrest%20PCP/Barrel%20Tensioner_zpsetigcsea.jpg.html)
Bob
That’s a great idea!
I bet the collar could have holes drilled and tapped 45 to existing, and new holes used as an outer shroud LDC anchor point.
Did you put shallow indents on the barrel for the set screws to push into and grab?
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I have used a variety of anchoring systems for the barrel, but most commonly use 6-8 setscrews into shallow pockets in the barrel to locate it.... I always make sure that the tensioning shroud slides over a stub on the receiver, to stiffen the joint at that point.... I don't want a baseball bat stiff barrel hinged at the receiver by a thin barrel.... Here is a much more fully developed system that I used on my .22 cal Disco Double....
(http://i378.photobucket.com/albums/oo221/rsterne/22%20cal%20Disco%20Double/Barrrel%20Tensioner%20Parts_zpsvunfq67a.jpg) (http://s378.photobucket.com/user/rsterne/media/22%20cal%20Disco%20Double/Barrrel%20Tensioner%20Parts_zpsvunfq67a.jpg.html)
This uses a barrel extension consisting of a small vented chamber that threads onto the muzzle.... There are four 1-caliber holes in the extension to vent some of the gasses into the shroud to aid in lowering the report.... I dubbed this the "Vented Barrel Tensioner" or VBT....
(http://i378.photobucket.com/albums/oo221/rsterne/22%20cal%20Disco%20Double/Vented%20Barrel%20Tensioner_zpspswrvq1w.jpg) (http://s378.photobucket.com/user/rsterne/media/22%20cal%20Disco%20Double/Vented%20Barrel%20Tensioner_zpspswrvq1w.jpg.html)
The shouldered collar behind the Bellevilles sits inside the shroud and pushes on the front end.... The "nut" is turned aluminum with flats for a wrench to tighten it and compress the Belleville stack.... It covers them to keep the dirt out....
(http://i378.photobucket.com/albums/oo221/rsterne/22%20cal%20Disco%20Double/Vented%20Tensioner%20Assembly_zpsc3ntz7kx.jpg) (http://s378.photobucket.com/user/rsterne/media/22%20cal%20Disco%20Double/Vented%20Tensioner%20Assembly_zpsc3ntz7kx.jpg.html)
I added a scale with 12 marks, like the hours on a clock, for a visible indication of the tension applied.... The Bellevilles chosen can provide nearly 1300 lbs. of tension, with about 2 turns of travel on the "nut".... so each mark is roughly 50 lbs. of tension.... Typical tension runs 800-1100 lbs., and the tension helps keep the extension aligned with the bore.... The threads on the front of the extension, where the adjusting nut ride, could be lengthened to fit an LDC if desired....
(http://i378.photobucket.com/albums/oo221/rsterne/22%20cal%20Disco%20Double/Tension%20Vernier_zpsuf9ipnrv.jpg) (http://s378.photobucket.com/user/rsterne/media/22%20cal%20Disco%20Double/Tension%20Vernier_zpsuf9ipnrv.jpg.html)
Below you can see the long stub on the receiver that the sleeve fits over to help support the assembly.... and some of the 8 setscrews that take the tension load on the barrel....
(http://i378.photobucket.com/albums/oo221/rsterne/22%20cal%20Disco%20Double/Bolt%20and%20Band_zpspnnqlj9k.jpg) (http://s378.photobucket.com/user/rsterne/media/22%20cal%20Disco%20Double/Bolt%20and%20Band_zpspnnqlj9k.jpg.html)
The only problem is that my lack of time in the summer has prevented proper accuracy testing.... Just as a harmonic tuner requires adjustment, it is likely that a tensioner such as this will also have a sweet spot.... What I am hoping is that it is wide enough to allow changes in velocity or bullet without retuning.... The advantage of the Belleville stack is that it provides a much smaller percentage change in tension as the temperature changes.... Without them, you would have to use the same material for the shroud and barrel to minimize that effect....
Bob
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Let us know how the accuracy test go. I found very light tension worked best on my experiment, of course that was the last thing I tried.
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Can we do the thing where this stays at the top? Will someone second the motion?
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You mean a "sticky".... I think that now we have gathered many important threads in the Workshop Gate, they should be easy to find by entering the Workshop and then searching.... for instance in this case just the words "Barrel Harmonics".... I don't want to have too many stickies so that you don't see the current threads....
How about one sticky as an Index that we can add links to as requested.... Lloyd and I can decide what belongs in the Index....
Bob
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That’s a good idea.
I’ve used the search engine before with the exact words, did not work to well.
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............ I don't want to have too many stickies so that you don't see the current threads....
How about one sticky as an Index that we can add links to as requested.... Lloyd and I can decide what belongs in the Index....
Bob
Bob, an index sounds interesting and useful, and I agree about keeping the number of stickies to a minimum. Possibly have several categories in the index, with threads listed under each one. There would probably be overlap between categories for topics that didn't fit cleanly into a single category.
I don't know if this is a good idea or not, but it might be worth slightly changing the title of some topics if the direction of the thread shifted and a different title might be more descriptive.
Lloyd
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Now that all our important threads are in one Gate (the Workshop), it should be a piece of cake to find anything of interest.... Just enter the Workshop, and then Search for keywords.... You get not only titles, but entries within threads with those keywords.... I think this means we don't really need an index.... but it may help people find threads.... However, it could be a PITA to set up and keep current.... not a high priority for me right now....
Bob
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;) ;D I'm sure you and Lloyd are much happier causing members like me serious headaches reading through your threads 3 or 4 times before the light switches on in our heads . keep up the good work 8) 8)