Positive compensation vs Sine wave tuning?

[jamesdmock]

Years ago I read an article by HOF shooter, noted gunsmith and action manufacturer Jim Borden about tuning a 6PPC using the Sine wave to determine charge weights. In his tests an accurate rifle was fired at a series on targets that were at the same height on the paper. The charge weights were varied by a few tenths and plotted on the chart. After many charge weights were tried, a sine wave was applied to the shots. The best two loads were those that fell just before the lowest point on the wave and just after the highest point. This was tuning for short range BR.

Is the positive compensation that I read about for long range related to Jim's test in any way? How is it different? Thanks for any information. James

 

[jimmymac]

Nothing to add other than, I'd like to read that article.

 

[Northridge]

Years ago I read an article by HOF shooter, noted gunsmith and action manufacturer Jim Borden about tuning a 6PPC using the Sine wave to determine charge weights. In his tests an accurate rifle was fired at a series on targets that were at the same height on the paper. The charge weights were varied by a few tenths and plotted on the chart. After many charge weights were tried, a sine wave was applied to the shots. The best two loads were those that fell just before the lowest point on the wave and just after the highest point. This was tuning for short range BR.

Is the positive compensation that I read about for long range related to Jim's test in any way? How is it different? Thanks for any information. James

yes this does relate to barrel compensation , but you will have to find premium location or compensation at all yardages it will not be linear from 100 yards to 1000 yards. the biggest different up close is closer yardages will mask problems that will show up at 1000 yards or any given yardage, dropped shots, vertical stringing, sensitive horizontal stringing, Bench Manners etc. A side note people often say they don't have a premium range to do this and that is fine, "a cop out" but fine. you do not need a perfect setting to gather data for further tuning everything will just be larger but your info is still on target. The key is to document tendency's with in a given tune or ladder setting.

Shawn Williams

 

[CPorter]

Are you talking about this kind of tuning?

http://www.the-long-family.com/optimal barrel time.htm

 

[BoydAllen]

When you have positive compensation, lower velocity bullets exit the barrel when it is a slightly higher point of its upswing than higher velocity bullets. The higher barrel position compensates for the trajectory of the slightly lower velocity, reducing vertical dispersion. If the bullets exit past the peak in the barrels rise, the reverse is true and vertical caused by differences in velocity will be exaggerated. Varmint Al showed that there are three ways to achieve positive compensation. One would be to hang a weight on the end of the barrel. I believe that for his simulation and later test it was an 8 oz tuner. The barrel was a 6PPC of a length typical for short range benchrest. Another way it could be done was to make the barrel longer, something that is typical of long range rifles. The third was to reduce the diameter of the middle of the barrel, making it a little more limber. All of these methods slowed the rise of the muzzle so that bullets would exit while it was rising. The goal was to have them exit just before it peaked so that the rate of rise was less.

 

[mikecr]

Are you talking about this kind of tuning?
http://www.the-long-family.com/optimal barrel time.htm

Different animals.
Positive compensation is about bullet timing w/resp to barrel flex, as droop is taken out under pressure.
OBT is about bullet timing w/resp to bore dimension changes, as caused by firing vibrations.
A good load timing would likely put bullet exit when the muzzle is moving least, and stable in bore dimension.
But there is still powder & pressure & rest influences separately steering our final results/choices

 

[KenOath]

Varmint Al's predictions were based off short range rifles. Hanging a weight on the barrel is not proving to help pc like we hoped, the opposite may be true on a long range rifle. I am convinced that center of gravity and flex in the forend are the best ways to achieve pc in a long range rifle. You can go too far, but I have not found that point yet. I have build experimental stocks to test theory, extreme flex and played with c/g to test out the ideas. This is my favorite topic. I am convinced that PC is the reason you have guys who shoot smaller than ES will allow, its the only way that going to happen. Want to shoot 1" at 1k? Aint happening without PC. All those ideas are in the stocks Mcmillan is making now. So many people believe you need a stiff forend, and thats been proven wrong. To compensate at short range is a whole different ball game, and I would not build a SR rifle the same way as a LR rifle.

I shoot my Dasher at 500m mostly from a bench and my 284 out to 900m when shooting F Class. Both my 284 and Dasher are in McMillan F Class stocks, I don't have anything to compare them to as both are my first competition guns but I've seen nothing to convince me that the stocks have to much flex in forend as has been stated in the past on numerous sites. Both guns have won and placed very well at State and National level, I'm guessing the flex may help with PC??

Cheers Rushty

 

[BradY]

Boyd is correct as per normal.

I have done load tuning before at short range looking at the sine waves produced by shooting round robin style ocw style tests. It was possible to pick a group in the right location then go straight to seating depth and tuner adjustments and see PC at extended range. Often the ES was around the 15-20 which I was told was too much. Targets don't lie though... beware if you get caught looking at one screamer group in the wrong spot you can get caught out chasing your tail especially further away.

I then added fixed dampener weights to the hv profile barrels and found the node to be increased in size but powder charge needed adjusting as it had moved in location.

 

[BoydAllen]

A long time ago, I did a test that involved adding a small fixed weight to a short range barrel, at the muzzle. I actually taped it in place (very securely) . With the weight the node was moved so that I could reach it with a few tenths less powder. To confirm I removed the weight and retuned. It returned to its previous charge weight. The reason that I mention this is that if this held true, a fellow could move a node down to a slightly lighter charge in cases where he had run out of room in the case before he got to it, or where pressure became an issue before he got to it. Instead of dropping all the way down to the next node, he could simply adjust the positions of the nodes slightly. This was just one test, which means that it is a long way from proven but it did work the way that I have described.