Does a flatter trajectory translate to a more accurate round?
- Thread starter Michael
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I have to admit I do not clearly understand how the spin rate was calculated with the screens and the ink. I am only asking from curiosity, not doubting the method of calculation.
As far as the 200 yard test: If I understand it correctly, a very accurately done 3-shot group served as an example of efficient FPS/spin rates which was translated into "twist rate" relationship? This test lead us to a range of velocities and twist rates for that particular pellet that performed excellent from 0 to 200 yards. So it showed us what works, and provided some insight into what may and may not work at longer ranges when using various FPS or "twist rates".
If I understand it correctly, the test showed that FPS decreases at a much higher rate than the RPM (spin rate) increases, and the relationship seems to be in a logarithmical fashion. Meaning that for each unit of FPS decrease the rate of increase of the spin rate (as translated into "twist rate") becomes less and less. If it would be possible (unlikely) to measure at much longer ranges (past 200 yards) theoretically one may reach a lower "spin rate" that causes pellet instability/inaccuracy? Since the ability to measure way past 200 yards seems very unlikely, could the shooter start at a lower MV using the same 200 yard range and try to establish a point between 0 and 200 yards where inaccuracy starts? If we start at a lower MV it seems that the spin rate deterioration may be able to reach a point where it may affect accuracy (using the same twist rate rifle as in the first test). Or would the spin rate ever get to a point where it affects accuracy before the pellet hits the ground (i,e, the logarithm curve stays near level way past the range of the rifle?
I did find a link with an equation, but it's for bullet and not pellets. So not sure if any other variables should also be considered for diablo pellets:
Bullet RPM = MV X 720/Twist Rate (in inches)
http://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/
As far as the 200 yard test: If I understand it correctly, a very accurately done 3-shot group served as an example of efficient FPS/spin rates which was translated into "twist rate" relationship? This test lead us to a range of velocities and twist rates for that particular pellet that performed excellent from 0 to 200 yards. So it showed us what works, and provided some insight into what may and may not work at longer ranges when using various FPS or "twist rates".
If I understand it correctly, the test showed that FPS decreases at a much higher rate than the RPM (spin rate) increases, and the relationship seems to be in a logarithmical fashion. Meaning that for each unit of FPS decrease the rate of increase of the spin rate (as translated into "twist rate") becomes less and less. If it would be possible (unlikely) to measure at much longer ranges (past 200 yards) theoretically one may reach a lower "spin rate" that causes pellet instability/inaccuracy? Since the ability to measure way past 200 yards seems very unlikely, could the shooter start at a lower MV using the same 200 yard range and try to establish a point between 0 and 200 yards where inaccuracy starts? If we start at a lower MV it seems that the spin rate deterioration may be able to reach a point where it may affect accuracy (using the same twist rate rifle as in the first test). Or would the spin rate ever get to a point where it affects accuracy before the pellet hits the ground (i,e, the logarithm curve stays near level way past the range of the rifle?
I did find a link with an equation, but it's for bullet and not pellets. So not sure if any other variables should also be considered for diablo pellets:
Bullet RPM = MV X 720/Twist Rate (in inches)
http://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/
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For fe and others who may wish to use the method: some further explanation re- the two screen (and three in controls) and inked pellet method of data resolution.
This is a very old method.
The pellet is marked such that as it passes through appropriate paper/light cardboard screens, it leaves a mark on the edge of the holes in both (or three) screens #1 and #2. The sheets have to be squared to each other..
As can be seen on the 200 yd test sheets, squared-up crosses are drawn for reference. Lines are then drawn across each pellet hole to dissect the hole, through the ink marks, using the ink marks for reference. On a 360d scale, the lines are given a degree rating. The difference in that rating between the front and rear screens is the indication of the degree of rotation the pellet undertook in the intervening gap between the screens. Then using the distance between the screens the rotation is resolved into the distance that one full 360d rotation would have undertaken.
The further apart the screens are the less the possible subjective errors in the assessment process. Thus in the pilot studies it is essential to choose an appropriate screen spacing. It is possible to put them so close together that less than one rotation is recorded; say 12 or 15 inches apart for a barrel that has a 1:17" rating. Or a spacing can be chosen that allows for something between one and two full rotations (or other). ... A pilot study should be done using three screens to ensure capturing the relevant data and not under or over estimating the number of rotations. In this case the screen separation was a little more than the barrel twist rate to allow one+ full revolution at the closest (known muzzle) range. The assumption was that that spacing would capture the effective spin rate within two full revolutions.
Ultimately the care with which all is set up and measured will be reflected in the degree of accuracy of the results. ... Is that not always the case in any laboratory? ... So the figures you see are the best reflection of my efforts which were to establish the trend for spin rates per foot of travel over the selected ranges.
Each of the ranges tested had the same treatment and results collated. The muzzle velocities were chronographed to ensure only shots of similar mv were used. The rifle has an extremely tight mv spread shot to shot.
" If I understand it correctly, the test showed that FPS decreases at a much higher rate than the RPM (spin rate) increases ..."
Yes. Spin rate defined as rotation per unit of travel. Actual radial velocity declines of course and is indicated in the text above.
In any extrapolation or interpolation regarding predictions based upon muzzle velocities in an attempt to make use of this information, that has also to be considered. ... This response classically shows how it is much more difficult to cover all the bases in a response/answer, than to ask a question.
..."could the shooter start at a lower MV using the same 200 yard range and try to establish a point between 0 and 200 yards where inaccuracy starts?" ... You are correct to assume from the 200 yd 3 shot group that with the pellet and rifle used accuracy was/is still excellent. On occasion 7 groups were shot with an average under 3" ; and the last 5 shot group at 300 yards was 7" with 4 shots under 4".
So in this case it might be assumed that 1:12.8" spin rate is still somewhat OK to 200 yards and these heavy high BC pellets have remained dynamically stable in the very good chosen conditions to 200 yards. Extreme range dynamic instability could be assumed to then occur at faster spin rates maybe 1:10 or whatever. To research that within 200 yards, using your model, would probably require muzzle spin rates that would begin at say 1:14" to overlap. That may require another barrel because the initial spin rate is equal to the barrel twist rate and apparently effectively climbs from there.
Lowering and raising the MV can certainly be reflected in lack of precision grouping at medium and longer range, Raising MV from a considered optimal MV can certainly indicate an MV that is too fast and that will induce spiral flight the nature of which is made worse be certain winds. I have done that test. A certain pellet I have will begin to show a % of spirals at 920 fps even in calm air (and lower MVs in wind); and increasing % at progressively higher MVs 'til at 980 fps all shots spiral. I presume there to be a link to the phenomenon under discussion. Other tests at progressively lower MVs, within reason and within stability criteria, have indicated initial velocities which produce stable flights regardless of conditions. So yes, this is all well worth playing with if one has the interest and time.
I will stop here as this is already too long for holding many readers interest and I don't wish to get far from what my study basically showed and into further conjecture and surmise. Hope you understand and that the first part was reasonably well explained. ... Best regards, Harry.
This is a very old method.
The pellet is marked such that as it passes through appropriate paper/light cardboard screens, it leaves a mark on the edge of the holes in both (or three) screens #1 and #2. The sheets have to be squared to each other..
As can be seen on the 200 yd test sheets, squared-up crosses are drawn for reference. Lines are then drawn across each pellet hole to dissect the hole, through the ink marks, using the ink marks for reference. On a 360d scale, the lines are given a degree rating. The difference in that rating between the front and rear screens is the indication of the degree of rotation the pellet undertook in the intervening gap between the screens. Then using the distance between the screens the rotation is resolved into the distance that one full 360d rotation would have undertaken.
The further apart the screens are the less the possible subjective errors in the assessment process. Thus in the pilot studies it is essential to choose an appropriate screen spacing. It is possible to put them so close together that less than one rotation is recorded; say 12 or 15 inches apart for a barrel that has a 1:17" rating. Or a spacing can be chosen that allows for something between one and two full rotations (or other). ... A pilot study should be done using three screens to ensure capturing the relevant data and not under or over estimating the number of rotations. In this case the screen separation was a little more than the barrel twist rate to allow one+ full revolution at the closest (known muzzle) range. The assumption was that that spacing would capture the effective spin rate within two full revolutions.
Ultimately the care with which all is set up and measured will be reflected in the degree of accuracy of the results. ... Is that not always the case in any laboratory? ... So the figures you see are the best reflection of my efforts which were to establish the trend for spin rates per foot of travel over the selected ranges.
Each of the ranges tested had the same treatment and results collated. The muzzle velocities were chronographed to ensure only shots of similar mv were used. The rifle has an extremely tight mv spread shot to shot.
" If I understand it correctly, the test showed that FPS decreases at a much higher rate than the RPM (spin rate) increases ..."
Yes. Spin rate defined as rotation per unit of travel. Actual radial velocity declines of course and is indicated in the text above.
In any extrapolation or interpolation regarding predictions based upon muzzle velocities in an attempt to make use of this information, that has also to be considered. ... This response classically shows how it is much more difficult to cover all the bases in a response/answer, than to ask a question.
..."could the shooter start at a lower MV using the same 200 yard range and try to establish a point between 0 and 200 yards where inaccuracy starts?" ... You are correct to assume from the 200 yd 3 shot group that with the pellet and rifle used accuracy was/is still excellent. On occasion 7 groups were shot with an average under 3" ; and the last 5 shot group at 300 yards was 7" with 4 shots under 4".
So in this case it might be assumed that 1:12.8" spin rate is still somewhat OK to 200 yards and these heavy high BC pellets have remained dynamically stable in the very good chosen conditions to 200 yards. Extreme range dynamic instability could be assumed to then occur at faster spin rates maybe 1:10 or whatever. To research that within 200 yards, using your model, would probably require muzzle spin rates that would begin at say 1:14" to overlap. That may require another barrel because the initial spin rate is equal to the barrel twist rate and apparently effectively climbs from there.
Lowering and raising the MV can certainly be reflected in lack of precision grouping at medium and longer range, Raising MV from a considered optimal MV can certainly indicate an MV that is too fast and that will induce spiral flight the nature of which is made worse be certain winds. I have done that test. A certain pellet I have will begin to show a % of spirals at 920 fps even in calm air (and lower MVs in wind); and increasing % at progressively higher MVs 'til at 980 fps all shots spiral. I presume there to be a link to the phenomenon under discussion. Other tests at progressively lower MVs, within reason and within stability criteria, have indicated initial velocities which produce stable flights regardless of conditions. So yes, this is all well worth playing with if one has the interest and time.
I will stop here as this is already too long for holding many readers interest and I don't wish to get far from what my study basically showed and into further conjecture and surmise. Hope you understand and that the first part was reasonably well explained. ... Best regards, Harry.
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Yes Dan, it's another topic, but you obviously noticed I took care to consider convention in my response above. Thank you for underlining that.
For others and to highlight your point here are the three mentions:
"Ultimately the care with which all is set up and measured will be reflected in the degree of accuracy of the results. … Is that not always the case in any laboratory?"
That is to say, in this experiment "accuracy" or "hitting the mark" depends upon careful measurements and data collation.
Then, responding to Fe's quote I wrote: "You are correct to assume from the 200 yd 3 shot group that with the pellet and rifle used accuracy was/is still excellent." ..
In that instance it can be noted from the picture that the 3 shot group on the screens at 200 yards was well centered on the screens. In other words it was hitting the intended spot which denotes accuracy. (It was also a fairly tight group so precision was also good. Thus, in terms of Emocon's chart, it not only had high accuracy but also high precision).
The final mention of these terms was .. "Lowering and raising the MV can certainly be reflected in lack of precision grouping at medium and longer range.. "
In that quote the implication was that precision can be affected at medium and longer ranges. But it also implies that you may not see much change at short ranges. It is not uncommon that when MV is pushed to even supersonic velocity, tight groups (precision) at 10 to even 25 yards may be quite good. But when dynamic instability triggers further down range it all flies out the window so to say.
Did you enjoy my responses and give them a tick, or were they just too long and detailed?.... Very best regards, Harry.
For others and to highlight your point here are the three mentions:
"Ultimately the care with which all is set up and measured will be reflected in the degree of accuracy of the results. … Is that not always the case in any laboratory?"
That is to say, in this experiment "accuracy" or "hitting the mark" depends upon careful measurements and data collation.
Then, responding to Fe's quote I wrote: "You are correct to assume from the 200 yd 3 shot group that with the pellet and rifle used accuracy was/is still excellent." ..
In that instance it can be noted from the picture that the 3 shot group on the screens at 200 yards was well centered on the screens. In other words it was hitting the intended spot which denotes accuracy. (It was also a fairly tight group so precision was also good. Thus, in terms of Emocon's chart, it not only had high accuracy but also high precision).
The final mention of these terms was .. "Lowering and raising the MV can certainly be reflected in lack of precision grouping at medium and longer range.. "
In that quote the implication was that precision can be affected at medium and longer ranges. But it also implies that you may not see much change at short ranges. It is not uncommon that when MV is pushed to even supersonic velocity, tight groups (precision) at 10 to even 25 yards may be quite good. But when dynamic instability triggers further down range it all flies out the window so to say.
Did you enjoy my responses and give them a tick, or were they just too long and detailed?.... Very best regards, Harry.
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Thank you Harry for the very detailed responses! I hope that your eyes are getting back to normal and the questions did not add too much strain when answering them. Unfortunately, AGN won't let me give you another accuracy point for such comprehensive answer, because I already did for one of your previous posts.
Now I understand how the 200 yard test with the ink marking is done and how it is measured. It's great to have a way to be able to read such difficult to measure variables, and the results seem to provide a very good indication of the trends and relationships. Such test must require a lot of preparation and precise setup, not even mentioning an accurate rifle. I am still working on my 50-100 yards accuracies, let alone 200 or 300 yards, but it's very interesting and challenging trying to undressing how one can deliver a small pellet with such extreme accuracy by understanding the variables behind it.
I wish more of these principles (such as those highlighted by the 200 yard test) would be incorporated into pellet and airgun designs. The JSB's high BC Heavy.25 pellet(s) seem to be engineered from the very onset to be a heavy hitting but flat trajectory pellet that must have been a conscious decision and not just an accidental design. Why is JSB on the cutting edge when airgun manufacturers would benefit the most having their own pellet-research department that would bring out the best of their products' performance? Airgun manufacturers seem to be content with making average performance pellets (Crosman Premier comes to mind) that perform well in a large variety of guns, but some manufacturers' own premier brand-name pellets are usually rebranded JSBs. Besides cost reasons, why is that JSB is arguably the best pellet maker in the world but they do not make any airguns? Maybe the pellet ballistics, twist rates, etc and other airgunning variables should be mapped out and matched to actual airguns with as much fervor as bringing new rifles on the market?
Now I understand how the 200 yard test with the ink marking is done and how it is measured. It's great to have a way to be able to read such difficult to measure variables, and the results seem to provide a very good indication of the trends and relationships. Such test must require a lot of preparation and precise setup, not even mentioning an accurate rifle. I am still working on my 50-100 yards accuracies, let alone 200 or 300 yards, but it's very interesting and challenging trying to undressing how one can deliver a small pellet with such extreme accuracy by understanding the variables behind it.
I wish more of these principles (such as those highlighted by the 200 yard test) would be incorporated into pellet and airgun designs. The JSB's high BC Heavy.25 pellet(s) seem to be engineered from the very onset to be a heavy hitting but flat trajectory pellet that must have been a conscious decision and not just an accidental design. Why is JSB on the cutting edge when airgun manufacturers would benefit the most having their own pellet-research department that would bring out the best of their products' performance? Airgun manufacturers seem to be content with making average performance pellets (Crosman Premier comes to mind) that perform well in a large variety of guns, but some manufacturers' own premier brand-name pellets are usually rebranded JSBs. Besides cost reasons, why is that JSB is arguably the best pellet maker in the world but they do not make any airguns? Maybe the pellet ballistics, twist rates, etc and other airgunning variables should be mapped out and matched to actual airguns with as much fervor as bringing new rifles on the market?
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