AR2217 in SAUM
Moderator: Mod
Re: AR2217 in SAUM
I am not sure if I mentioned it, but this saum has an 8.5 twist krieger.
id quod est
-
AlanF
- Posts: 7532
- Joined: Wed Jun 15, 2005 8:22 pm
- Location: Maffra, Vic
- Has thanked: 229 times
- Been thanked: 936 times
Re: AR2217 in SAUM
I don't have any first hand experience with either 280AI or 7 SAUM, but FWIW from what I READ, short fat cases burn powder more efficiently and with cooler primers. Also they allow the use of shorter (more rigid) actions, and in the case of SAUM, a large case head for longer brass life. The only advantage I see for the 280AI is availability of good brass (Lapua, RWS etc).
-
KHGS
- Posts: 950
- Joined: Thu Oct 20, 2005 12:46 am
- Location: Cowra NSW
- Has thanked: 776 times
- Been thanked: 537 times
Re: AR2217 in SAUM
AlanF wrote:I don't have any first hand experience with either 280AI or 7 SAUM, but FWIW from what I READ, short fat cases burn powder more efficiently and with cooler primers. Also they allow the use of shorter (more rigid) actions, and in the case of SAUM, a large case head for longer brass life. The only advantage I see for the 280AI is availability of good brass (Lapua, RWS etc).
There are other advantages Alan. I am NOT convinced that short fat BIG cases are all that good at all, that theory started with small bore small capacity cases e.g. 6mm BR & 6mm PPC. I believe that short fat big capacity cases which usually have shorter than desirable necks lead to steeper pressure curves creating higher primer pocket pressure & earlier throat erosion due to the larger diameter powder column. I do not think that it matters much how you stack 50 to 60 grains of slow burning powder the ignition requirements will be similar.
All the target actions we use will handle 30/06 length cases with no problem. The short magnum cases were designed to suit shorter repeating sporter actions so the OAL of the rifle could be kept to a minimum for easier carrying in the field. As I said earlier, I tend to not follow trends, so my thinking may be considered unorthodox!
Keith H.
-
williada
- Posts: 969
- Joined: Wed Jul 22, 2009 12:37 am
- Has thanked: 263 times
- Been thanked: 422 times
Re: AR2217 in SAUM
Keith, thanks for your comments, you are spot on with regards the velocity and staying within the suitable limits rather than being on the ragged edge. The very reason I did not get into lengthy discussion on the other thread as it would distract people from the main game of consistency.
To keep it brief, the twist rate does affect the resistance of the bullet to moving on or off line through its influence on torque. It also influences the Magnus force effect which affects the amount of spindrift both in elevation and windage and more so at different distances as the forward velocity fades if these factors are change. So it can screw your calculations unless you practice new ones for wind reading. The Magnus force also affects the Magnus moment at the centre of pressure and the centre of gravity of the projectile and the duration or length of the overturning point in the bullet trajectory. There is a lot to consider in the tradeoffs of using one setup over another. Better to keep it simple if you don't understand this stuff and run with gear successful people are using.
If you are on the ragged end with twist rates, either to get more stability in turbulent conditions; and this is why we see sometimes TR shooters, or F standard shooters get up with good scores with a 1-10 instead of 1-12 with 155’s, but it might not group as well in other conditions. Similarly, I remember shooting a 60.9 with a .308 in 1-15. I used it to reduce perceived bullet balance problems with jackets or cores – to minimise the affect of unbalanced projectiles opening up the group like a lawn bowl. It was initially designed for short range English Radway Green ammunition. It was brilliant out to 700 if conditions suited, but it had a narrow band of air density in which to perform with our stuff. Yes it was quicker, due to less friction but it also had less barrel lift so tuning it for maximum compensation was out of the question. That was not needed for short range. I also put in a 1/2 degree leed angle which further reduced lift and recoil. It felt beautiful to shoot, but sometimes it fell apart in the wrong air.
Some are indeed fortunate to select bullets for balance like Vaughn did through his dynamic air balancing tool or a static bullet balancer that he demonstrated in his book, “Rifle Accuracy Facts”. Others have used the Juenke concentricity machine. So there are clear advantages to people who are hush, hush about their gear.
Are they better shots?
The very reason I have written these articles which I am now trying to close the discussion on, is to empower the grass roots of the sport to keep the movement growing so that future competitors do not face as many barriers to entry and get browned off and leave. Strong competition on a level playing field will develop real champions who can read the conditions. I suppose turn the wheel of discussion is like re-inventing it.
Could I say how valuable it is for people such as Norm putting in comments about what he recently did to his barrel on another post, because there are others who can connect the dots to demonstrate where the topic fits in the big picture.
In order to manage something big and unwieldy, you have to break it into small bits first, to get a handle on it. With anything new, you have to storm the ideas. These often question your assumptions, but by testing things, you eventually conclude what is best so it becomes the norm (pardon the pun Norm). From there you have established a set of benchmarks to improve your performance. Then the bloody rules change or the goal post shifts and you start again. Bloody speed limits. David.
To keep it brief, the twist rate does affect the resistance of the bullet to moving on or off line through its influence on torque. It also influences the Magnus force effect which affects the amount of spindrift both in elevation and windage and more so at different distances as the forward velocity fades if these factors are change. So it can screw your calculations unless you practice new ones for wind reading. The Magnus force also affects the Magnus moment at the centre of pressure and the centre of gravity of the projectile and the duration or length of the overturning point in the bullet trajectory. There is a lot to consider in the tradeoffs of using one setup over another. Better to keep it simple if you don't understand this stuff and run with gear successful people are using.
If you are on the ragged end with twist rates, either to get more stability in turbulent conditions; and this is why we see sometimes TR shooters, or F standard shooters get up with good scores with a 1-10 instead of 1-12 with 155’s, but it might not group as well in other conditions. Similarly, I remember shooting a 60.9 with a .308 in 1-15. I used it to reduce perceived bullet balance problems with jackets or cores – to minimise the affect of unbalanced projectiles opening up the group like a lawn bowl. It was initially designed for short range English Radway Green ammunition. It was brilliant out to 700 if conditions suited, but it had a narrow band of air density in which to perform with our stuff. Yes it was quicker, due to less friction but it also had less barrel lift so tuning it for maximum compensation was out of the question. That was not needed for short range. I also put in a 1/2 degree leed angle which further reduced lift and recoil. It felt beautiful to shoot, but sometimes it fell apart in the wrong air.
Some are indeed fortunate to select bullets for balance like Vaughn did through his dynamic air balancing tool or a static bullet balancer that he demonstrated in his book, “Rifle Accuracy Facts”. Others have used the Juenke concentricity machine. So there are clear advantages to people who are hush, hush about their gear.
Are they better shots?
The very reason I have written these articles which I am now trying to close the discussion on, is to empower the grass roots of the sport to keep the movement growing so that future competitors do not face as many barriers to entry and get browned off and leave. Strong competition on a level playing field will develop real champions who can read the conditions. I suppose turn the wheel of discussion is like re-inventing it.
Could I say how valuable it is for people such as Norm putting in comments about what he recently did to his barrel on another post, because there are others who can connect the dots to demonstrate where the topic fits in the big picture.
In order to manage something big and unwieldy, you have to break it into small bits first, to get a handle on it. With anything new, you have to storm the ideas. These often question your assumptions, but by testing things, you eventually conclude what is best so it becomes the norm (pardon the pun Norm). From there you have established a set of benchmarks to improve your performance. Then the bloody rules change or the goal post shifts and you start again. Bloody speed limits. David.
-
williada
- Posts: 969
- Joined: Wed Jul 22, 2009 12:37 am
- Has thanked: 263 times
- Been thanked: 422 times
Re: AR2217 in SAUM
I had some discussion privately about group analysis which I would like to share.
People are under the impression that you can get tighter groups at long range if you know how to tune for it.
To put it bluntly, you cannot make a silk purse out of a sow’s ear. If the barrel is not capable of grouping tightly enough, between the cessation of the coning distance, (say 140 yards, to eliminate meaningful dynamic instability) and the overturning moment, there is no magic fix to recover any angular dispersion at long range.
It’s a separate issue, and a matter of probability at these shorter distances whether the rifle’s best tune is tighter than the outlying shots that make up that tune. Or, if indeed you have had sufficient shots to be representative of the true group size. A minimum statistical sample size is 30 to be of significance. That is good reason to examine successive trends in groups, not that some estimates can’t be an educated guess, but it is technically a guess if you are using less than 30 shots by commonly accepted standards. Whatever that tune at short range, the size of the group generally won’t get better with our equipment at long range due to stiff barrels as I will explain shortly.
That being said, the most consistent section of the bullet’s flight which is going to reveal the barrel’s true potential is the short range testing as already indicated. In this situation, and within reason, the bullet will maintain a given trajectory for a given velocity. Conditions are better for testing at shorter distances because results are not distorted as much by temperature, wind and humidity.
I have found that the general integrity of groups is maintained with increasing distance. However, beyond the overturning point of the projectile the groups generally enlarge, but remain in formation if conditions remain constant. The marksmanship then becomes the management of a larger group. Of course your tune at closer distances has to be as tight as possible to give you more freedom at the longer ranges. You have to shrink those outliers in your tune as is possible through load development.
The group opens up, but retains shape, in its downward flight past the trajectory peak because of the greater nose wobble or yaw created by the shift in the centre of pressure in relation to the magnus force in the flight transition from overturning moment, together with gravity taking charge due to falling velocity. Here, the bullet’s spin decay is insignificant, so it will maintain its path about a central plane as determined by the bullet’s velocity and it will keep nudging its nose into the direction of the wind flow although wobbling and slightly expanding the group shape due to increased drag of the nose taking a wider arc.
Whether your tune is based on a nodal tune or an optimum charge weight tune is irrelevant if it is your best tune, if we assume conditions remain constant. What is important is reducing the deviations of velocity with your best tune because this minimises further dispersion in elevation at long range.
Positive compensation can help you manage the larger group at long range but only in the vertical plane because the force of gravity is down. Given the stiff barrels we use, means that positive compensation is at best minimal if it can be achieved in load development; and for it to work given our small target centre, the group size has still to be bound by a tight velocity range. Therefore you cannot turn a bad barrel into a good one, but you can enhance a good one at long range with it still being handy at shorter distances if slow shots exit the muzzle in a higher position than fast shots. This assumes the projectile’s exit is timed with a vertical swing of the muzzle, not a lateral one or combination of directions.
If you have a neutral barrel which is representative of many OCW tunes where all bullets are tossed out on the same angle vertically, it is hard to distinguish whether target hits in terms of elevation are due to velocity variance. However, vertical spread seems to be expressed at very long ranges e.g. 1000 yards with minor velocity spreads with velocity variation caused by such things as poor primers. This variation in velocity at long range will be expressed in the vertical with fast bullets falling on top of slow bullets unless your barrel is not indexed properly for the load and it tosses shots at say 11 or 1 o'clock.
If slow bullets are released well below fast bullets in the downward sweep of a barrel, the negative compensation profile of the barrel will mean bullets will disperse vertically at each incremental distance. If the SD’s are very tight, you would expect that with no significant velocity variation the trajectories would be the same. But you can be fooled into thinking a tight negative compensation tune at short range will hold, because invariably the climatic conditions vary and so does your velocity which then causes your group to expand in typical negative fashion.
Most ballistic programs, which model trajectory and group dispersion in particular, as do some physicists, actually mislead shooters because they assume the bullet launch angle is constant. In reality, we know this is not true because of different barrel compensation profiles. The assumption of constant launch angle under simulation conditions implies fast bullets exit on top of slow bullets and their models are based on that. That is a negative compensation profile! That means slow bullets won’t travel as far.
This means for example, a simulated plot on a target of say 100 shots which is truly statistically significant if it was relevant is not truly representative of the group dispersion for a given SD.
Anybody who has fought a fire with poor hose pressure will tell you can increase the distance the water hits if you raise the hose angle. This is just the same as positive compensation.
The programs usually talk in degrees for muzzle angle, but there are 60 minutes to one degree. As F Class shooters, we need to hit the major target of 1 minute of angle, so subtle changes in compensation profile are very, very relevant for long range. Sure you can fudge things to get programs to alter target height to fiddle around the edges. The only way to I know to make a simulation realistic is to do it yourself on Microsoft Excel where you can vary launch angle in minutes of angle with varying velocities to see where trajectories intersect.
In terms of practical application, the groups at long range are always going to be subject to elevation dispersion due to increasing wind from the left or the right due to spin drift in addition to yaw factors previously mentioned.
Suffice it to say, the epicyclical swerve of the bullet path has both vertical and horizontal components; and the group also naturally enlarges but holds shape if you are able to discount environmental factors as well. Therefore I don’t think a positive compensation tune will be generally tighter under the same conditions of testing at long range compared to a short range tune. It can be pretty close to it. But I am very sure that a neutral and indeed a negatively compensating barrel will have group dispersion at long range greater than your short range tune.
I also agree with Keith's earlier reasoning with regard to large cases. The role of slow burning powder does not require the instant snap of ignition in the powder column. I think what is of more importance is the circulation and turbulence control of escaping gases that steeper shoulders such as Ackley Improved give. David.
People are under the impression that you can get tighter groups at long range if you know how to tune for it.
To put it bluntly, you cannot make a silk purse out of a sow’s ear. If the barrel is not capable of grouping tightly enough, between the cessation of the coning distance, (say 140 yards, to eliminate meaningful dynamic instability) and the overturning moment, there is no magic fix to recover any angular dispersion at long range.
It’s a separate issue, and a matter of probability at these shorter distances whether the rifle’s best tune is tighter than the outlying shots that make up that tune. Or, if indeed you have had sufficient shots to be representative of the true group size. A minimum statistical sample size is 30 to be of significance. That is good reason to examine successive trends in groups, not that some estimates can’t be an educated guess, but it is technically a guess if you are using less than 30 shots by commonly accepted standards. Whatever that tune at short range, the size of the group generally won’t get better with our equipment at long range due to stiff barrels as I will explain shortly.
That being said, the most consistent section of the bullet’s flight which is going to reveal the barrel’s true potential is the short range testing as already indicated. In this situation, and within reason, the bullet will maintain a given trajectory for a given velocity. Conditions are better for testing at shorter distances because results are not distorted as much by temperature, wind and humidity.
I have found that the general integrity of groups is maintained with increasing distance. However, beyond the overturning point of the projectile the groups generally enlarge, but remain in formation if conditions remain constant. The marksmanship then becomes the management of a larger group. Of course your tune at closer distances has to be as tight as possible to give you more freedom at the longer ranges. You have to shrink those outliers in your tune as is possible through load development.
The group opens up, but retains shape, in its downward flight past the trajectory peak because of the greater nose wobble or yaw created by the shift in the centre of pressure in relation to the magnus force in the flight transition from overturning moment, together with gravity taking charge due to falling velocity. Here, the bullet’s spin decay is insignificant, so it will maintain its path about a central plane as determined by the bullet’s velocity and it will keep nudging its nose into the direction of the wind flow although wobbling and slightly expanding the group shape due to increased drag of the nose taking a wider arc.
Whether your tune is based on a nodal tune or an optimum charge weight tune is irrelevant if it is your best tune, if we assume conditions remain constant. What is important is reducing the deviations of velocity with your best tune because this minimises further dispersion in elevation at long range.
Positive compensation can help you manage the larger group at long range but only in the vertical plane because the force of gravity is down. Given the stiff barrels we use, means that positive compensation is at best minimal if it can be achieved in load development; and for it to work given our small target centre, the group size has still to be bound by a tight velocity range. Therefore you cannot turn a bad barrel into a good one, but you can enhance a good one at long range with it still being handy at shorter distances if slow shots exit the muzzle in a higher position than fast shots. This assumes the projectile’s exit is timed with a vertical swing of the muzzle, not a lateral one or combination of directions.
If you have a neutral barrel which is representative of many OCW tunes where all bullets are tossed out on the same angle vertically, it is hard to distinguish whether target hits in terms of elevation are due to velocity variance. However, vertical spread seems to be expressed at very long ranges e.g. 1000 yards with minor velocity spreads with velocity variation caused by such things as poor primers. This variation in velocity at long range will be expressed in the vertical with fast bullets falling on top of slow bullets unless your barrel is not indexed properly for the load and it tosses shots at say 11 or 1 o'clock.
If slow bullets are released well below fast bullets in the downward sweep of a barrel, the negative compensation profile of the barrel will mean bullets will disperse vertically at each incremental distance. If the SD’s are very tight, you would expect that with no significant velocity variation the trajectories would be the same. But you can be fooled into thinking a tight negative compensation tune at short range will hold, because invariably the climatic conditions vary and so does your velocity which then causes your group to expand in typical negative fashion.
Most ballistic programs, which model trajectory and group dispersion in particular, as do some physicists, actually mislead shooters because they assume the bullet launch angle is constant. In reality, we know this is not true because of different barrel compensation profiles. The assumption of constant launch angle under simulation conditions implies fast bullets exit on top of slow bullets and their models are based on that. That is a negative compensation profile! That means slow bullets won’t travel as far.
This means for example, a simulated plot on a target of say 100 shots which is truly statistically significant if it was relevant is not truly representative of the group dispersion for a given SD.
Anybody who has fought a fire with poor hose pressure will tell you can increase the distance the water hits if you raise the hose angle. This is just the same as positive compensation.
The programs usually talk in degrees for muzzle angle, but there are 60 minutes to one degree. As F Class shooters, we need to hit the major target of 1 minute of angle, so subtle changes in compensation profile are very, very relevant for long range. Sure you can fudge things to get programs to alter target height to fiddle around the edges. The only way to I know to make a simulation realistic is to do it yourself on Microsoft Excel where you can vary launch angle in minutes of angle with varying velocities to see where trajectories intersect.
In terms of practical application, the groups at long range are always going to be subject to elevation dispersion due to increasing wind from the left or the right due to spin drift in addition to yaw factors previously mentioned.
Suffice it to say, the epicyclical swerve of the bullet path has both vertical and horizontal components; and the group also naturally enlarges but holds shape if you are able to discount environmental factors as well. Therefore I don’t think a positive compensation tune will be generally tighter under the same conditions of testing at long range compared to a short range tune. It can be pretty close to it. But I am very sure that a neutral and indeed a negatively compensating barrel will have group dispersion at long range greater than your short range tune.
I also agree with Keith's earlier reasoning with regard to large cases. The role of slow burning powder does not require the instant snap of ignition in the powder column. I think what is of more importance is the circulation and turbulence control of escaping gases that steeper shoulders such as Ackley Improved give. David.
Menu
Newest members