How Barrel Harmonics Can Affect POI

[rsterne]

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)....



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....



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"....



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")....



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

[SpiralGroove]

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.

[rsterne]

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

[Kailua]

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.

[rsterne]

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

[skorec]

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 ? 

[rkr]

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?

[Sfttailrdr46]

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 

[rsterne]

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....



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

[Sfttailrdr46]

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 ??

[whitefox545]

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.
                                                                                                   

[SpiralGroove]

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....



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 .

[rsterne]

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....



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....



On these barrels I will leave a threaded stub at the muzzle, and install an adjustable tuner....



Bob

ADDENDUM.... Here are photos of the finished Harmonic Tuners I made for my 6 mm and .257 mentioned in this post....





Steel sleeve about 2.5 oz, brass weight just over 5 oz.... No results as yet....

RBS

[grand-galop]

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..

[rsterne]

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

[whitefox545]

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.

[rsterne]

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

[SpiralGroove]

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................

[whitefox545]

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.

[plinker81366]

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?