Hey... long rangers... got a bullet / predictability question...
- Thread starter Magnum Wheel Man
- Start date
some interesting reading, with pictures of bullets in flight.
http://www.dtic.mil/dtic/tr/fulltext/u2/a205633.pdf
The M118 bullet had the shallowest boat tail at 9.3 degrees. The 190 SMK had an 11 degree boat tail, and the 168 SMK had the infamous 13 degree boat tail. The picture of the 168 at mach 1.1 is especially telling when you compare it to the M118 or 190 SMK at the same speed.
Jimro
http://www.dtic.mil/dtic/tr/fulltext/u2/a205633.pdf
The M118 bullet had the shallowest boat tail at 9.3 degrees. The 190 SMK had an 11 degree boat tail, and the 168 SMK had the infamous 13 degree boat tail. The picture of the 168 at mach 1.1 is especially telling when you compare it to the M118 or 190 SMK at the same speed.
Jimro
Jimro said:
I'm curious what you see.
The quality of the pics aren't all that good, but to me, it looks like there's more turbulence behind the 168s. And the rear wave seems to become less parallel to the front wave as it gets close to the bullet itself.
My WAG is that "cubic Magnus moments" are a measure of yaw at a given speed. Seems they're all pretty stable at supersonic speeds, but the 168 "moments" jump more at subsonic speeds. I'm not enough of an engineer to know whether this means more or less yaw, or tease out what all this means for transonic stability. So, is the 168 worse than the others?
I do note that the center of gravity of the 168s is closer to the geometric center than the others. So, again, is the 168 better or worse when going transonic?
Mr. Borland,
The 168 is worse in transonic stability.
Note the curved second shockwave near the base of the 168 where the M118 and 190 SMK are still very straight. That shockwave is blending into the rear drag turbulance area as the steep 13 degree boat tail has a larger turbulent area than the shallower boat tails of the other two bullets.
Jimro
The 168 is worse in transonic stability.
Note the curved second shockwave near the base of the 168 where the M118 and 190 SMK are still very straight. That shockwave is blending into the rear drag turbulance area as the steep 13 degree boat tail has a larger turbulent area than the shallower boat tails of the other two bullets.
Jimro
First off Bart and I have shot the same matches at the same time but never knew each other and we both have seen elongated holes more times than we care to remember with 168s at 1000 yards. What makes it worse is we both know/knew the same people which has in all probability ruined both our reputations haha.
I had a interesting experience/observation in the 86 Palma Match with the 168s. I got squadded with a Gunny I had never met so we started shooting and I caught my favorite relay for the Palma (1st) and I observed he was doing well with the stick and I had a 30.06 Mod 70.
We went to the pits and of course after 800 his holes were round but that afternoon we shot the 900 and 1000 and went to the pits. I had a good score that I thought would be beat and the Gunny had a good score as well and wasn't far behind me score wise.
In the pits guys were reporting their scores and my hopes raised a bit and after firing was completed we got on the carts for the ride to the line and Mid Tompkins was on the other side of cart and asked me how many I was down and I told him and he said, "That is the best I have heard." But as Bart and I both know when they squad the Palma the High Masters are squadded by the luck of the card they hand you.
We both fired the 900 and 1000 and went to the pits and though his holes were elongated he still had a good score so we started talking and we started talking about 168s yawing and he said in their mount testing at Quantico they had come to the conclusion that the transition did not affect their accuracy that much so they stayed with 168s and since his score was very close to mine I could not challenge the evidence.
I got back to the firing line and was going to get my cart/gear and ran into George Tubb who was taking up score cards and he looked at me and smiled and said, "Looks like you got it again." I had won the Palma Match the previous year with same rifle/load. But the surprise was yet to come.
At the award ceremony they make awards for high bolt gun and high service rifle and to both our surprises he won high service rifle catagory and they called out the bolt gun winner and George was right, I had taken it again.
Talk about odds, what the odds of two guys that never met, getting squadded together on the same point and both of us winning our separate catagories.
At that time I was the only person to ever win the Palma two times, now it appears eight others are on the list but with several years separations between their wins. There are two morals to this thread, first is elongated bullets can still give you a good score and once you hit a certain level in long range competition it is not guarantee you will win every time because if you are shooting when you are supposed to scratching and/or scratching when you should have been shooting makes the difference between being on top that day or near the top.
I had a interesting experience/observation in the 86 Palma Match with the 168s. I got squadded with a Gunny I had never met so we started shooting and I caught my favorite relay for the Palma (1st) and I observed he was doing well with the stick and I had a 30.06 Mod 70.
We went to the pits and of course after 800 his holes were round but that afternoon we shot the 900 and 1000 and went to the pits. I had a good score that I thought would be beat and the Gunny had a good score as well and wasn't far behind me score wise.
In the pits guys were reporting their scores and my hopes raised a bit and after firing was completed we got on the carts for the ride to the line and Mid Tompkins was on the other side of cart and asked me how many I was down and I told him and he said, "That is the best I have heard." But as Bart and I both know when they squad the Palma the High Masters are squadded by the luck of the card they hand you.
We both fired the 900 and 1000 and went to the pits and though his holes were elongated he still had a good score so we started talking and we started talking about 168s yawing and he said in their mount testing at Quantico they had come to the conclusion that the transition did not affect their accuracy that much so they stayed with 168s and since his score was very close to mine I could not challenge the evidence.
I got back to the firing line and was going to get my cart/gear and ran into George Tubb who was taking up score cards and he looked at me and smiled and said, "Looks like you got it again." I had won the Palma Match the previous year with same rifle/load. But the surprise was yet to come.
At the award ceremony they make awards for high bolt gun and high service rifle and to both our surprises he won high service rifle catagory and they called out the bolt gun winner and George was right, I had taken it again.
Talk about odds, what the odds of two guys that never met, getting squadded together on the same point and both of us winning our separate catagories.
At that time I was the only person to ever win the Palma two times, now it appears eight others are on the list but with several years separations between their wins. There are two morals to this thread, first is elongated bullets can still give you a good score and once you hit a certain level in long range competition it is not guarantee you will win every time because if you are shooting when you are supposed to scratching and/or scratching when you should have been shooting makes the difference between being on top that day or near the top.
Last edited:
Jimro, thanks for posting that link to some very interesting stuff. I plan to go through it more than my quick glance did, but I noticed one interesting thing.
They used reduced charges of IMR4198 for lower muzzle velocities to "duplicate" down range in-flight behaviour of all three bullets. From what I've gleaned from several sources, bullet spin rate drops 10% to 15% from its rpm number at the muzzle to what it is going through the target at 1000 yards. If Mach 1 is 1125 fps, a bullet leaving at Mach 2.219 from a 1:12 twist barrel will spin 149,782.5 rpm. From a 1:10 twist barrel, it's rpm number goes up to 179,739 rpm. The same bullet leaving at Mach .703 will spin a bit over 3/10ths as fast; 47,452.5 rpm from a 1:12 twist and 56,943 rpm from a 1:10.
Those slow bullet launch velocities, to me, mean their spin rate at the velocity they'll have way down range are much lower than normally fired bullets will have. Would that change the results?
They used reduced charges of IMR4198 for lower muzzle velocities to "duplicate" down range in-flight behaviour of all three bullets. From what I've gleaned from several sources, bullet spin rate drops 10% to 15% from its rpm number at the muzzle to what it is going through the target at 1000 yards. If Mach 1 is 1125 fps, a bullet leaving at Mach 2.219 from a 1:12 twist barrel will spin 149,782.5 rpm. From a 1:10 twist barrel, it's rpm number goes up to 179,739 rpm. The same bullet leaving at Mach .703 will spin a bit over 3/10ths as fast; 47,452.5 rpm from a 1:12 twist and 56,943 rpm from a 1:10.
Those slow bullet launch velocities, to me, mean their spin rate at the velocity they'll have way down range are much lower than normally fired bullets will have. Would that change the results?
Bart B.,
That's a good observation, and I think that the lower spin doesn't matter in this case because the spark diagrams are looking at "laminar flow" (except the projectile is moving, not the fluid) interactions.
The reduced reduced spin wouldn't change the spark diagrams that give you a look at the aerodynamic interactions. For all intents and purposes spin is simply the stabilizing factor that allows a bullet to nose into the wind, a slower spin simply means more precession yaw earlier in the flight.
Since we know that yaw precession grows as velocity drops and rpms decrease, the pictures of the bullets as is is a good indication of what projectiles at full distance are dealing with in terms of aerodynamics across the surface of the projectile. As long as there is enough spin for stability, the pictures are indicative of aerodynamics.
Jimro
That's a good observation, and I think that the lower spin doesn't matter in this case because the spark diagrams are looking at "laminar flow" (except the projectile is moving, not the fluid) interactions.
The reduced reduced spin wouldn't change the spark diagrams that give you a look at the aerodynamic interactions. For all intents and purposes spin is simply the stabilizing factor that allows a bullet to nose into the wind, a slower spin simply means more precession yaw earlier in the flight.
Since we know that yaw precession grows as velocity drops and rpms decrease, the pictures of the bullets as is is a good indication of what projectiles at full distance are dealing with in terms of aerodynamics across the surface of the projectile. As long as there is enough spin for stability, the pictures are indicative of aerodynamics.
Jimro
Hummer70's comment about when one shoots while on the line with other equally ranked competitors separates the best scores from the darned near the best.
Best example of that to me was in the 1993 Rocky Mountain Palma Matches at Raton. In the last of 4 days of those long range matches, I was ahead of those just below me by several points shooting the final match stage. At 1000 yards with the high masters on the line, half way through it, a big wind gust suprised everyone. Depending on when you shot, the 155's were blown different distances to the left. Those that shot ahead of that gust had bullet holes on paper anywhere from 10 inches to 36 inches left of center and stayed on paper in the scoring rings and those at the edge were scored 6 points. Those who shot when it hit had their bullets go more than 36 inches from center; off the paper and were scored a miss; 10 points down. I finished third over all with a miss losing 10 points as did several others. Mid and Nancy Tompins finished ahead of me by 2 and 3 points respectively; they shot ahead of that gust. Had I been at the edge of the paper, I would have been 6 points higher and ahead of them by a few points.
There's some luck involved in all the shooting sports. Which is why the NRA changed the firing point assignments on the 1000-yard Viale Range at Camp Perry. The first 20 some odd firing points on the west side next to the tree line had scores fired on them typically higher than those at points 21 through 99 further east. It was a reason to level the playing field that all thought was a good thing to do.
Best example of that to me was in the 1993 Rocky Mountain Palma Matches at Raton. In the last of 4 days of those long range matches, I was ahead of those just below me by several points shooting the final match stage. At 1000 yards with the high masters on the line, half way through it, a big wind gust suprised everyone. Depending on when you shot, the 155's were blown different distances to the left. Those that shot ahead of that gust had bullet holes on paper anywhere from 10 inches to 36 inches left of center and stayed on paper in the scoring rings and those at the edge were scored 6 points. Those who shot when it hit had their bullets go more than 36 inches from center; off the paper and were scored a miss; 10 points down. I finished third over all with a miss losing 10 points as did several others. Mid and Nancy Tompins finished ahead of me by 2 and 3 points respectively; they shot ahead of that gust. Had I been at the edge of the paper, I would have been 6 points higher and ahead of them by a few points.
There's some luck involved in all the shooting sports. Which is why the NRA changed the firing point assignments on the 1000-yard Viale Range at Camp Perry. The first 20 some odd firing points on the west side next to the tree line had scores fired on them typically higher than those at points 21 through 99 further east. It was a reason to level the playing field that all thought was a good thing to do.