Inaccuracy node?

[FatOldMIHunter]

Are there pressure nodes where extreme inaccuracy occurs? Suppose you're doing load development and varying the load by .5 grains with each group. The vertical spread is small, say 1-1.5 MOA for all the groups except one, which goes to 2.5 or more. You repeat the group and the groups +/- .5 grains to be certain and the results are the same. What would you expect to be the cause of that?

 

[BoydAllen]

Just out of curiosity, what kind of rifle in what caliber, shot at what distance?

 

[FatOldMIHunter]

Just out of curiosity, what kind of rifle in what caliber, shot at what distance?

Whatever you are shooting at whatever distance you do load development at.

 

[BoydAllen]

First of all, you are asking for an explanation of the entire theory of why accuracy nodes exist, and the specific mechanisms involved in a reply on a forum on the internet. While you may get some good suggestions on how to solve a specific problem your question is so general that the answer would exceed the length that a typical poster would want to put in the time required to write. You are asking for several chapters of detailed information. Good luck with that....really

 

[FatOldMIHunter]

First of all, you are asking for an explanation of the entire theory of why accuracy nodes exist, and the specific mechanisms involved in a reply on a forum on the internet. While you may get some good suggestions on how to solve a specific problem your question is so general that the answer would exceed the length that a typical poster would want to put in the time required to write. You are asking for several chapters of detailed information. Good luck with that....really

And narrowing it down to a specific rifle, caliber, and distance would somehow change that?

I understand that maximum accuracy is presumed to occur when the bullet exits the barrel at the end of an oscillation. So wouldn't it be as simple as assuming that the maximum inaccuracy occurs when the bullet exits at the zero crossing?

 

[BoydAllen]

And narrowing it down to a specific rifle, caliber, and distance would somehow change that?

I understand that maximum accuracy is presumed to occur when the bullet exits the barrel at the end of an oscillation. So wouldn't it be as simple as assuming that the maximum inaccuracy occurs when the bullet exits at the zero crossing?

Groups are formed by the impacts of multiple shots that do not have identical velocities, some positions on the muzzle rise plot are more forgiving with regard to these variations. There is also a theory that says that there is an expansion of the bore that travels up and back until the bullet exits and that if it happens to be at the muzzle at the same time that the bullet exits that accuracy is adversely affected. As a practical matter, too much theory can get in the way of simply doing experiments and taking good notes. Fellows waste a lot of time and often never come to a good result. The simple truth is that every thing that they think but have no real way to measure is simply a guess and if they guess wrong they can waste a lot of time trying to reconcile an incorrect theory with data that will never match it. Working up a load is really not that complicated, but lots of fellows fall short of the mark because they try to do too many things at once, and spend way too much time on the internet and not nearly enough time loading and shooting at the range, using wind flags. It amazes me the extent to which the internet has confused so many people.

 

[nakneker]

Groups are formed by the impacts of multiple shots that do not have identical velocities, some positions on the muzzle rise plot are more forgiving with regard to these variations. There is also a theory that says that there is an expansion of the bore that travels up and back until the bullet exits and that if it happens to be at the muzzle at the same time that the bullet exits that accuracy is adversely affected. As a practical matter, too much theory can get in the way of simply doing experiments and taking good notes. Fellows waste a lot of time and often never come to a good result. The simple truth is that every thing that they think but have no real way to measure is simply a guess and if they guess wrong they can waste a lot of time trying to reconcile an incorrect theory with data that will never match it. Working up a load is really not that complicated, but lots of fellows fall short of the mark because they try to do too many things at once, and spend way too much time on the internet and not nearly enough time loading and shooting at the range, using wind flags. It amazes me the extent to which the internet has confused so many people.

There’s a whole bunch of truth in that paragraph.

 

[FatOldMIHunter]

Groups are formed by the impacts of multiple shots that do not have identical velocities, some positions on the muzzle rise plot are more forgiving with regard to these variations. There is also a theory that says that there is an expansion of the bore that travels up and back until the bullet exits and that if it happens to be at the muzzle at the same time that the bullet exits that accuracy is adversely affected. As a practical matter, too much theory can get in the way of simply doing experiments and taking good notes. Fellows waste a lot of time and often never come to a good result. The simple truth is that every thing that they think but have no real way to measure is simply a guess and if they guess wrong they can waste a lot of time trying to reconcile an incorrect theory with data that will never match it. Working up a load is really not that complicated, but lots of fellows fall short of the mark because they try to do too many things at once, and spend way too much time on the internet and not nearly enough time loading and shooting at the range, using wind flags. It amazes me the extent to which the internet has confused so many people.

I confess that I don't shoot often around midnight, but are you implying that if this happened to you, you would think, "Man, I've been spending too much time on the Internet."? Why would you use wind flags when you're focusing on vertical spread during load development? What do you use for updraft or downdraft indicators during load development or otherwise when you shoot?

Let's go back to my original question. What would you expect to be the cause of an inaccuracy blip? Would you just simply shrug your shoulders and move on to where you achieved highest accuracy?

 

[Berger.Fan222]

More likely an antinode.

 

[BoydAllen]

I tend to use fixed procedures that have proven to get me to a good load in the fewest number of shots. Your remark about wind flags shows that you have not had much experience shooting over a set, which is no crime or flaw, but in the short range benchrest game we look at our groups and not simply vertical. At the level that we are working, the wind has significant effects on vertical even without up or down drafts. I do my testing over flags on days when the wind is relatively easy, load at the range and can usually work up a load in a relatively small number of shots. Discussions of one to one and a half moa of vertical are just completely out of my range of error. What I am shooting for is a small two and on a very good day, with luck, five shots in perhaps a high .1xx. If I have any paper at all between two shots of a preliminary test I assume that I am at the worst spot between two nodes and make a suitable correction. If groups get into the high twos to low threes for five shots when the wind has not caused it (something that flags can tell you) I retune, trying a couple of variations. Long range shooters tune differently, but none of them that I know would find one to one and a half moa of vertical acceptable.

 

[dmoran]

@FatOldMIHunter
Myself refer to them as "scatter nodes" and find them very useful that aid in pointing out the best increments and direction. As to creating pressure, unless at max pressure already, they really are not much different and not a pressure creator or risk - IME.
Donovan

 

[dgeesaman]

The theory of the pulse that travels up and down the barrel is helpful here.

Have a look at the "heartbeat" looking graph halfway down this page: http://www.the-long-family.com/OBT_paper.htm
That graph shows the diameter of the bore at the crown during a firing cycle. What is helpful to know is accuracy is worst at the moments where the pulse is reflecting off the crown end of the barrel, where the barrel goes from oversize to undersize almost instantly. This gives solid theory for scatter nodes. The rest of the paper boils down to a math-y way to tweak your loads to avoid that heartbeat and find the stable area away from it.

 

[FatOldMIHunter]

The theory of the pulse that travels up and down the barrel is helpful here.

Have a look at the "heartbeat" looking graph halfway down this page: http://www.the-long-family.com/OBT_paper.htm
That graph shows the diameter of the bore at the crown during a firing cycle. What is helpful to know is accuracy is worst at the moments where the pulse is reflecting off the crown end of the barrel, where the barrel goes from oversize to undersize almost instantly. This gives solid theory for scatter nodes. The rest of the paper boils down to a math-y way to tweak your loads to avoid that heartbeat and find the stable area away from it.

That paper is fascinating. I will call them scatter nodes. Thank you.

 

[FatOldMIHunter]

I tend to use fixed procedures that have proven to get me to a good load in the fewest number of shots. Your remark about wind flags shows that you have not had much experience shooting over a set, which is no crime or flaw, but in the short range benchrest game we look at our groups and not simply vertical. At the level that we are working, the wind has significant effects on vertical even without up or down drafts. I do my testing over flags on days when the wind is relatively easy, load at the range and can usually work up a load in a relatively small number of shots. Discussions of one to one and a half moa of vertical are just completely out of my range of error. What I am shooting for is a small two and on a very good day, with luck, five shots in perhaps a high .1xx. If I have any paper at all between two shots of a preliminary test I assume that I am at the worst spot between two nodes and make a suitable correction. If groups get into the high twos to low threes for five shots when the wind has not caused it (something that flags can tell you) I retune, trying a couple of variations. Long range shooters tune differently, but none of them that I know would find one to one and a half moa of vertical acceptable.

Sorry, I've been assuming a load development procedure similar to Erik Cortina's where wind is not factored into the initial load development. That clearly doesn't apply to you.

 

[BoydAllen]

If you are developing loads for long range use then copy those that shoot at those ranges. One of the things that I see is fellows asking about load development who seem to have other rifle/shooting/ support system problems that need to be solved before they go off on their quest for a load. We have a lot of fellows out there messing with rifles that have not had their actions bedded and barrels floated, shooting barrels that may not be worth bothering with, and/or scope issues. I suggest that you look at why you are getting so much vertical. The reason that I asked about your equipment and distance is that things that matter at long range matter a lot less at short range. For instance, at long range uniformity of velocity is very important and shows up as vertical, while this is much less the case at short range. We cannot put wind flags out every few yards in the path of the flight of the bullet when doing load development at long range, and we can at short range, so long range shooters have to adapt to that reality in how they work up loads. You asked a very general question and blew me off when I asked for specifics, possibly not realizing that as with many things the best answer for a specific situation depends on those sorts of details. If you can come up with a load that has very low ES at a usable velocity working at short range, you will probably be able to tune it with seating depth to get what you need at long range. But if your are tuning for 1-200 yard benchrest, you will be working differently. ES is not nearly as important and you will be able to see what the wind is doing and actually look at groups rather than just vertical.

 

[Bart B.]

And narrowing it down to a specific rifle, caliber, and distance would somehow change that?

I understand that maximum accuracy is presumed to occur when the bullet exits the barrel at the end of an oscillation. So wouldn't it be as simple as assuming that the maximum inaccuracy occurs when the bullet exits at the zero crossing?

i don't think everyone thinks maximum accuracy (precision?) occurs wnen bullets leave at the end (top or bottom) of its whip cycle. If all bullets leave about the top of their vertical cycle, the higher velocity half will leave on the small upswing (shorter barrel time) to the top and the lower velocity half will leave on the small downswing (longer barrel time). If average velocity bullets leave at the equidistant extremes, the fastest and slowest bullets leave at the same angle relative to the line of sight.

If all bullets leave on the muzzle axis upswing, the lower velocity ones leave at a higher angle than the faster ones. If all leave on the downswing, the reverse happens.

Where should all bullets leave so both slower and higher velocity ones get the best compensation for their drop downrange?

 

[Bart B.]

Have a look at the "heartbeat" looking graph halfway down this page: http://www.the-long-family.com/OBT_paper.htm
That graph shows the diameter of the bore at the crown during a firing cycle. What is helpful to know is accuracy is worst at the moments where the pulse is reflecting off the crown end of the barrel, where the barrel goes from oversize to undersize almost instantly.

That OBT paper is a theory.

Would the absence of copper wash at the last several fractions of an inch at the muzzle prove the muzzle expanded and its presence prove it did not?

Look at mode 4 barrel shape in
http://www.varmintal.com/amode.htm

I've had Garand barrels with no copper wash the last 3/4th inch of the bore at the muzzle that shot all rounds sub MOA through 600 yards. Bore and groove diameters had enlarged over .001 inch from new.

 

[FatOldMIHunter]

If you are developing loads for long range use then copy those that shoot at those ranges. One of the things that I see is fellows asking about load development who seem to have other rifle/shooting/ support system problems that need to be solved before they go off on their quest for a load. We have a lot of fellows out there messing with rifles that have not had their actions bedded and barrels floated, shooting barrels that may not be worth bothering with, and/or scope issues. I suggest that you look at why you are getting so much vertical. The reason that I asked about your equipment and distance is that things that matter at long range matter a lot less at short range. For instance, at long range uniformity of velocity is very important and shows up as vertical, while this is much less the case at short range. We cannot put wind flags out every few yards in the path of the flight of the bullet when doing load development at long range, and we can at short range, so long range shooters have to adapt to that reality in how they work up loads. You asked a very general question and blew me off when I asked for specifics, possibly not realizing that as with many things the best answer for a specific situation depends on those sorts of details. If you can come up with a load that has very low ES at a usable velocity working at short range, you will probably be able to tune it with seating depth to get what you need at long range. But if your are tuning for 1-200 yard benchrest, you will be working differently. ES is not nearly as important and you will be able to see what the wind is doing and actually look at groups rather than just vertical.

My question was about why someone would suddenly get a large amount of vertical when they were consistently getting a much smaller amount. In asking that, I was hoping to learn something interesting in a way that I'm accustomed to, asking questions of interest and seeing where that leads. In your case, that's not working toward anything that seems like it will be relevant to me. I asked a very specific question, "What would you expect to be the cause of a sudden dramatic decrease in accuracy (during a search for OCW)?". I'm not asking you to troubleshoot a problem I'm having (which seems to be your assumption). You've explained why the question isn't relevant to you and I'm okay with your answer. No further comment is necessary.

 

[FatOldMIHunter]

i don't think everyone thinks maximum accuracy (precision?) occurs wnen bullets leave at the end (top or bottom) of its whip cycle. If all bullets leave about the top of their vertical cycle, the higher velocity half will leave on the small upswing (shorter barrel time) to the top and the lower velocity half will leave on the small downswing (longer barrel time). If average velocity bullets leave at the equidistant extremes, the fastest and slowest bullets leave at the same angle relative to the line of sight.

If all bullets leave on the muzzle axis upswing, the lower velocity ones leave at a higher angle than the faster ones. If all leave on the downswing, the reverse happens.

Where should all bullets leave so both slower and higher velocity ones get the best compensation for their drop downrange?

Interesting, but it doesn't seem to address my original question. What would you expect to be the cause of a sudden dramatic decrease in accuracy during a search for OCW?

 

[Bart B.]

Interesting, but it doesn't seem to address my original question. What would you expect to be the cause of a sudden dramatic decrease in accuracy during a search for OCW?

The shooter changed the way he held the rifle.