How Barrel Harmonics Can Affect POI

[mpbby]

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 

[Saddlemountain]

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

[Horatio]

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.

[rsterne]

Exactly why I use a Belleville stack at the muzzle to minimize tension change with temperature when tensioning a barrel....



Bob

[A.K.A. Tommy Boy]

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

[lloyd-ss]

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 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.

[Horatio]

Exactly why I use a Belleville stack at the muzzle to minimize tension change with temperature when tensioning a barrel....



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?

[rsterne]

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....



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....



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....



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....



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....



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

[Saddlemountain]

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.

[Horatio]

Can we do the thing where this stays at the top? Will someone second the motion?

[rsterne]

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

[Horatio]

That’s a good idea.

I’ve used the search engine before with the exact words, did not work to well.

[lloyd-ss]

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

[rsterne]

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

[Sfttailrdr46]

  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