Positive compensation

[Keith Glasscock]

This question is for everyone that has done Bryan's shoot-through challenge or something similar.

Theoretically, a 1K range-based tune would be able to utilize the barrel movement to somewhat compensate for velocity variations.

Has anyone seen this done and the resultant effects at other ranges?

For example, my calculations show that with a .370 G7 BC, a velocity variation of 10 fps (average 2800), if fully compensated at 1k (no vertical), would give .2" of vertical at 100 yards and .7" at 600 yards.

What experimentation has been done, and what results have been obtained?

I'm considering undertaking a project on this.

 

[ballison]

This is one of the reasons I use ladder tests when developing loads. A faster bullet leaving the barrel at a lower point will have a similar POI to a slower bullet leaving higher. It will not be as obvious at 100 yds, but at longer distances it can stick out like a sore thumb. Some may say this has is a very minimal effect, but when you are talking about shooting 600-1000 yards, small effects add up. There are a lot of shooters (REALLY good shooters) that use OCW to develop loads, but I could never see enough of a vertical difference to use that method.

Attached is a pretty detailed document I found that explains how and why this works. It's not the end all, be all, but it has worked for me.

 

[Alex Wheeler]

First, positive compensation is fact, its been proven. Without it groups in the 1's at 1k just would not be possible with all the variations. I have seen a few guns that routinely shoot groups in the 1s for vertical at 1k.
Read this, dont just skim over it.
http://www.geoffrey-kolbe.com/articles/rimfire_accuracy/tuning_a_barrel.htm

 

[normmatzen]

First, while Dr. Kolbe is a sharp guy and builds some really interesting measuring equipment, he is a Rim Fire guy 100%.
In fact, he states that his research does NOT work for center fire.

I and my Australian friend have been doing a lot of work with tuners and have found that positive compensation, while possible at 1000 yd, is very difficult to obtain. In fact, most of the competent literature claims it is not possible. I personally think it is JUST possible with a good 6mm like the 6 Dasher or 6BRX. I have not been able to get quite there with my 284 WIN.
This may be why a lot of absolute group records at 1000 yds were shot with a 6 Dasher or 6BRX. Larger guns can do well with score, but not so good with group. Same with my 284 WIN.
Probably why 284 WIN is popular for F class but not for Bench Rest.

 

[gunsandgunsmithing]

First, while Dr. Kolbe is a sharp guy and builds some really interesting measuring equipment, he is a Rim Fire guy 100%.
In fact, he states that his research does NOT work for center fire.

I and my Australian friend have been doing a lot of work with tuners and have found that positive compensation, while possible at 1000 yd, is very difficult to obtain. In fact, most of the competent literature claims it is not possible. I personally think it is JUST possible with a good 6mm like the 6 Dasher or 6BRX. I have not been able to get quite there with my 284 WIN.
This may be why a lot of absolute group records at 1000 yds were shot with a 6 Dasher or 6BRX. Larger guns can do well with score, but not so good with group. Same with my 284 WIN.
Probably why 284 WIN is popular for F class but not for Bench Rest.

I see it as being less absolute than that. I think you can have some degree of compensation relatively easily, but 100% being nearly or even completely unobtainable. That said, any is better than none. The majority of rf shooters, including Calfee, don't think it's even anything more than a figment of our imagination and that scientists and engineers are wrong about it.


I absolutely do believe it happens and have seen it testing custom barrel contours. The theory here was to "hinge" a short stiff barrel by reducing it's diameter several inches behind the muzzle. I simply made a flat about 2" long x .125" deep at 8" behind the muzzle. The idea is to make the tune window wider and the barrel more sensitive to tuner weight..hence a lighter needed tuner weight to achieve the same effect. I can say that it has merit. The reasoning is making weight easier while using a tuner on LV rifles at 10.5lbs...or any other weight, for that matter.

 

[Keith Glasscock]

My concern in experimenting to try to identify positive compensation would be separating that effect from BC variation, wind, chronograph error (velocity variation unaccounted for), mirage, etc..

I think a realistic test would require a substantial number of rounds once a "tune" is selected. Using a tuner, the experiment could be repeated for various "tune" settings to see if it works as advertised.

 

[Alex Wheeler]

My concern in experimenting to try to identify positive compensation would be separating that effect from BC variation, wind, chronograph error (velocity variation unaccounted for), mirage, etc..

I think a realistic test would require a substantial number of rounds once a "tune" is selected. Using a tuner, the experiment could be repeated for various "tune" settings to see if it works as advertised.

Keith, no need to test. When you have a 15-20 fps es, that says you should have 3-4" of vertical not including conditions, bc variation, gun handling exc. but you just shot a 2-3" group thats all the proof you need. See it all the time. To find the right spot, shoot at your intended distance 600 or 1000. Find a spot where a velocity increase does not make a point of impact change. There you have it. It only works at one distance.

 

[normmatzen]

zfastmalibu,

I read all I can find about barrel tuners. Most of it anecdotal evidence.
We carry, along with our carefully loaded ammunition, a number of cartridge pairs with one group 1% or so higher load and the other 1% or so lower load. We color the bullets with dry-erase pens red and green for hi and low. We can then check out tune by shooting a pair at what ever range we are currently shooting.
At the last club 1000 yd match, I was not sure of my tuner setting as the temp had dropped. I shot one pair as sighters with the current tuner setting and another with a 1/40th turn adjustment and the mis-matched pair shot 3" at 1000 where the original setting was 10"!
If I had been shooting at the range for practice, you can see which round was which as the slight color ring is visible.
At 300 yd positive comp is very obvious and at 1000 not so much, but my 6mm does better than the 284 Win.
You are right tho, you can usually get close enough so the tuner is set on the correct slope of the "Sine" wave for VERY good compensation.

 

[normmatzen]

gunsandammunition,

Your idea for modifying the barrel is brilliant!

My friend and I both think there are folks out there, like armament designers, that have done some work including like you describe.
Personally, I don't have the equipment, skill or MONEY to do this kind of research, but it is sure interesting! I wonder how many military rifles with perturbations in the surface of the barrel that seem to be random or manufacturing conveniences.. Maybe not?
My friend and I have been trying to find electrical analogs of the barrel harmonics so we can simulate tuning with circuit simulation soft ware. Similar to what Varmint Al did with FEA simulation. But, we are electronic engineers, not mechanical engineers.

 

[dmoran]

..........The theory here was to "hinge" a short stiff barrel by reducing it's diameter several inches behind the muzzle. I simply made a flat about 2" long x .125" deep at 8" behind the muzzle. The idea is to make the tune window wider and the barrel more sensitive to tuner weight..hence a lighter needed tuner weight to achieve the same effect.

Mike

Very interesting input..... Thanks

Do you think "barrel fluting" behind/before the tuner, could act as a "hinge"?
If so, how long of flutes would you consider?
(in my case it would be for +28" HV barrels <> 600/1000-BR)

Same question again, but for "octagon'ing a section"?


Great thread.... and great replies!.!.!
Donovan

 

[gunsandgunsmithing]

..........The theory here was to "hinge" a short stiff barrel by reducing it's diameter several inches behind the muzzle. I simply made a flat about 2" long x .125" deep at 8" behind the muzzle. The idea is to make the tune window wider and the barrel more sensitive to tuner weight..hence a lighter needed tuner weight to achieve the same effect.

Mike

Very interesting input..... Thanks

Do you think "barrel fluting" behind/before the tuner, could act as a "hinge"?
If so, how long of flutes would you consider?
(in my case it would be for +28" HV barrels <> 600/1000-BR)

Same question again, but for "octagon'ing a section"?


Great thread.... and great replies!.!.!
Donovan

Thanks Donovan...I have no doubt that flutes make a barrel less stiff for its weight. I've not tested it but I see no reason why a short section of either flutes or octagonal flats wouldn't have a similar effect. Machining a round journal was just very easy yet effective means to an end. Flats or flutes would almost certainly be somewhat less so, but nonetheless, still would make it less stiff than without them. I hope that doesn't start the debate over fluted barrels being stiffer than non-fluted ones...they are not..given the same weight. I hope we all understand that.
I would like to see a time in the near future where we can move to some sort of barrel stiffness index when discussing tuners and such, as that appears to be the only constant. An 1.250 straight is not necessarily sztiffer than a HV or even a LV contour. Length affects stiffness more so than barrel OD.

Remember the stocks will play a role also. The whole system works together, at least that's the plan.

Tom

I think that's exactly right! I do have some ideas that I plan to to test soon. edit: On another thread, a couple of people have responded to questions about a "lead Sled". They both agreed that POI can be quite different with one than without. On that note, I've seen consistent differences in velocity between shooting with a gun pinned hard against a stop vs totally free recoil. I think it's virtually impossible for the bullet to reach the muzzle while it's still pointing EXACTLY in the same place if the same load can produce different velocities based on how the gun is allowed to move under recoil. The same goes for how tight the front rest is against the gun, the weight of the gun, shooting uphill or downhill...it all matters and in this way, is directly relative to tune, to some degree.

gunsandammunition,

Your idea for modifying the barrel is brilliant!

My friend and I both think there are folks out there, like armament designers, that have done some work including like you describe.
Personally, I don't have the equipment, skill or MONEY to do this kind of research, but it is sure interesting! I wonder how many military rifles with perturbations in the surface of the barrel that seem to be random or manufacturing conveniences.. Maybe not?
My friend and I have been trying to find electrical analogs of the barrel harmonics so we can simulate tuning with circuit simulation soft ware. Similar to what Varmint Al did with FEA simulation. But, we are electronic engineers, not mechanical engineers.

Norm, I credit Boyd Allen with the thought. IIRC, we discussed its potential merits for various reasons..weight being one, but yes,making the barrel respond more like a "less stiff barrel" was also discussed. Testing it did show me enough evidence to conclude that the tune windows are wider, and a lighter tuner was needed to give the same effect as a heavier one...or..smaller adjustments to give the same affect as with a heavier tuner and stiffer barrel. IOW, it seemed to work as planned. We must be careful to not assume that just because this teory panned out, that a heavier tuner and/or a less stiff barrel are the end all, be all. There is however, a fairly wide range of appropriate tuner weight to barrel stiffness ratio window...but you can have too much or too little of either, IME.

 

[gunsandgunsmithing]

Mike,

I find your input on getting different velocity for different "holds" very interesting. Why do you think this happens? I assume this was a benchrest rifle?

Tom

Simple Tom...a small amount of energy that would otherwise be pushing a bullet done the bore is expended pushing the gun back. The guns ability to move freely simply dissipates a small amount of energy. Try it sometime. It's easiest to see with a good LV due to good tracking and lighter weight. The amount of difference is usually small, about 10 fps, but it doesn't really matter. What matters is that it will bee consistently slower if free recoiled than if pinned hard. For the same reason, the whole gun would also vibrate at a different frequency, too. I strongly believe that a gun must break away and be free to move a small amount. .consistently. .to shoot well. Tests have been done where barrels were cast into concrete to hold the barrel very rigid and of course no ability to move. They won't shoot like that. It likely is more to due with harmonics than anything..but if the whole gun is held rigidly, the barrel would certainly vibrate with more magnitude than if some energy was allowed to release in the form of the gun re coiling rearward.

 

[gunsandgunsmithing]

Someone might actually get me to set a crony in front of one of my dashers! I'll have to make sure no one sees that.

Now on a serious note. I kind of knew your answer there. But it has me wondering, along the lines of why our light guns shoot so great. But every time I put together a heavy, while it may shoot some tiny groups, it seems to go away quicker. There has to be a way to have the right properties of positive compensation, and also have a tracking machine, as heavies are.

This winter I'm having Alex make me a 25+-lb dasher, on a normal stock, and normal hv taper. We'll see how that works out, as I'm sick of testing 50+ pound heavy guns. Especially when they never see a match because they won't beat my light guns lol.

Tom

 

[Ggmac]

I'm not sure anyone has mentioned it yet , but the British have known and used positive compensation for a long time . It seems there flexing SMLE rifles need much less elevation ( about half of a mauser , or Springfield ) at 1000 yds . It's due to the slower vel bullets leaving the barrel at a higher position than the faster . Precision shooting excellent book - shooting at 1000 , has some good articles on it.