Through our testing, we’ve learned that adequate spin-stabilization is important to achieving the best BC (and lowest drag). In other words, if you don’t spin your bullets fast enough (with sufficient twist rate), the BC of your bullets may be less than optimal. That means, in practical terms, that your bullets drop more quickly and deflect more in the wind (other factors being equal). Spin your bullets faster, and you can optimize your BC for best performance.
The scatter in the data and the R squared value indicate that only about 1/2 the variation in MV is due to twist rate (Correlation Coefficient is 0.55) which means that random noise has as much effect as twist rate. This is discussed further in the book, as well as similar results presented for a different bullet in which the relationship was even weaker, and the correlation was lower.
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Not truly correct.
BC is dependent on bullet Velocity as in G7 model.
Twist rate would set the stability factor. The more stable the better for accuracy.
What he is really saying that get your bullet stable (sb >= 1.5) before you shoot or all bets are off.
Beiruty,
United we stand, dispersed we falter
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Through our testing, we’ve learned that adequate spin-stabilization is important to achieving the best BC (and lowest drag). In other words, if you don’t spin your bullets fast enough (with sufficient twist rate), the BC of your bullets may be less than optimal. That means, in practical terms, that your bullets drop more quickly and deflect more in the wind (other factors being equal). Spin your bullets faster, and you can optimize your BC for best performance.
Edit: His point is not that spin-stabilization doesn't affect the BC, but that over-stabilization can result in a lower maximum effective range, which is contrary to the article's apparent blanket "more spin is better" claim. I don't have the equipment to test his claims.
I am not a lawyer, nor have I played one on TV, nor did I stay at a Holiday Inn Express last night, nor should anything I say be taken as legal advice. If it is important that any information be accurate, do not use me as the only source.
Once sf is >1.5 it means that your bullet is stable and for heavier bullets stability is essential. Lighter bullet not that much. as they would be already stable. Not sure about the side effects of over stabilization if any.
Beiruty,
United we stand, dispersed we falter
2014: NRA Endowment lifetime member
Beiruty wrote:Once sf is >1.5 it means that your bullet is stable and for heavier bullets stability is essential. Lighter bullet not that much. as they would be already stable. Not sure about the side effects of over stabilization if any.
Assuming he's right, the effect of over-stabilization (or at least the one I remember him talking about) is that the bullet can't turn to follow the downward leg, which destabilizes its trajectory.
Again, that assumes the guy is right. When he talks about what will have an effect on a bullet's trajectory, he makes a lot of sense to me. However I lack the real-world experience and/or math & physics background to say if he's correct in terms of how much of an effect all these factors have. Keep in mind that he's talking about making 1000+ yard shots on coke cans, so even if he's right, you can't blame missing the 10-ring at the pistol bay on over-stabilization.
I am not a lawyer, nor have I played one on TV, nor did I stay at a Holiday Inn Express last night, nor should anything I say be taken as legal advice. If it is important that any information be accurate, do not use me as the only source.
BC changes as both a function of spin and velocity.
The main point here is the effect of spin velocity on the spin axis as the bullet travels through its arc toward the target. It is true that, in a .308 for instance, a tighter twist will stabilize heavier bullets better than a looser twist, but that doesn't mean that one is good and one is bad; rather it means that one is better suited for a certain range of bullet weights, and the other is better suited for a different range of bullet weights. And THAT is really all the shooter needs to concern him/herself with. BC is not only going to be a product of forward velocity and spin, but it is also a product of bullet design. There is a reason that, regardless of manufacturer, centerfire match bullets are almost universally designed as boat-tail hollowpoints, and that reason is that, per caliber, that bullet design has the best combination of high BC and low aerodynamic drag. Given two bullets of equal caliber and weight, one a boat-tail hollowpoint and the other a flat-base spitzer, the boat-tail will maintain its velocity longer and carry further on the same powder charge than the flat-base bullet. The goal for most shooters should be to stick to common wisdom and don't worry too much about the arcana of G1 or G7 profiles, etc., etc. It would be almost impossible to spin a .308 Winchester bullet of any commonly used bullet weight so fast that it would come apart in flight with a barrel twist of between 1:10 and 1:12 inclusive.......which probably covers 99.9999999% of all .308 Winchester rifles ever manufactured. Ever. It would ALSO be nearly impossible for one of these rifles to spin a bullet so fast that it could not start to nose down after apexing its arc of travel. So this is a pointless thing to worry about. Instead, worry about what range of bullet weights would probably work best in the rifling twist yours has, and then find the optimum load that gives you the best combination of velocity and accuracy. In the process, be prepared to accept some things. Just because a 150 grain .308 bullet can safely exit the muzzle of a 24" 1:12 barrel at 2,820 fps, that doesn't mean that attempting the same velocity from a 1:10 with a 175 grain bullet won't blow my face apart. Accept that the 175 grain bullet going only 2,600 fps from the muzzle will be more stable and carrying more velocity at 1,000 yards down range than that 150 grain bullet that left the muzzle 2,820 fps will be doing at that distance.
My rule of thumb is lighter bullets for slower twists, and heavier bullets for faster twists. In my 1:9 AR15, I run bullets from 45-69 grains. In my 1:8 AR15 I run bullets from 60-77 grains. Ditto with the 1:7 SBR I'm building, although I might try some heavier subsonics. In my 1:12 .308 SCAR, I run bullets in the 147-168 grain range. In my 1:10 .308 bolt rifles, I run bullets in the 165-180 grain range.
The other reason besides spin-drift that too much spin can detract from accuracy is that it has a gyroscopic effect along the bullet's axis. Trajectory is a product of the barrel angle, bullet mass, bullet velocity, and gravity, and it happens regardless of what the bullet is doing along its axis. If that gyroscopic effect is still too strong as the trajectory reaches its apex begins to arc down, the bullet's nose will remain pointed upward and not turn downward toward the target...... such that the bullet is almost setting up for a belly-landing, instead of arcing downward, creating more of a keyhole entry than a point-first entry. If you are long-range hunting, that may mean the difference between a clean kill or not.
But the fact is that, unless you are running a magnum cartridge or some kind of large-cased necked-down wildcat cartridge, it would be hard to get most common rifles in the most common calibers to achieve so much spin from barrel twist as to to either frag the bullet in flight, or to put it into a "gyroscopic lock" where it couldn't respond organically to its flight path.
Velocity is good for ballistics for these reasons: it pumps up BC, it extends range, and it negates Coriolis force. (The quicker your bullet gets there, the less the earth will move in rotation underneath it during its flight.)
BC is good because it imparts stability to the bullet's flight, and it extends the bullet's effective range.
Spin is good because it stabilizes the bullet and is one of the variables affecting BC.
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Through our testing, we’ve learned that adequate spin-stabilization is important to achieving the best BC (and lowest drag). In other words, if you don’t spin your bullets fast enough (with sufficient twist rate), the BC of your bullets may be less than optimal. That means, in practical terms, that your bullets drop more quickly and deflect more in the wind (other factors being equal). Spin your bullets faster, and you can optimize your BC for best performance.
Edit: His point is not that spin-stabilization doesn't affect the BC, but that over-stabilization can result in a lower maximum effective range, which is contrary to the article's apparent blanket "more spin is better" claim. I don't have the equipment to test his claims.
Let me preface my remark by saying that I posted this as information...one person's perspective based on experiment...and have no dog in the fight. After practicing engineering for over 30 years and encountering many "experts," some generally acclaimed, some self-proclaimed, I remain uniformly unimpressed. Whenever I hear expert before a report or presentation I know that what follows will contain an argument from authority, and sometimes the legitimate part of the argument will support the claims, sometimes it won't. Usually, the weaker the argument the more emphasis on the credentials of the person making it. That also applies when someone is making a cogent argument that refutes the "experts" and and lacks their credentials and recognition.
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Yes it is. And if it is chocolate and has almonds it is even more awesomer!
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Physics-wise, once the spin rate was high enough to provide gyroscopic stability for the mass of the bullet, that rate would have already overcome any aerodynamic effects, and the Reynolds Number is so high that the boundary layer is turbulent anyway, so the drag coefficients are fixed.
Since it takes energy to spin the bullet, that is energy lost from making muzzle velocity, so it would seem to me any spin over the minimum does no good, and wastes energy.
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rbwhatever1 wrote:Do they make "Boat Tail" ice cream?
Yes, usually wrapped around a banana.
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I am not a lawyer, nor have I played one on TV, nor did I stay at a Holiday Inn Express last night, nor should anything I say be taken as legal advice. If it is important that any information be accurate, do not use me as the only source.
Having reread the thread and posts related, I can see where this might be important, as someone said, shooting at coke cans at 1000 yards, but not really for us day to day handgun carriers. I can't imagine that a millimeter or two difference in placement would have a great deal of effect on a BG 25 feet away.
OTOH, when I was shooting long range and trying to find the best combination of metals, the right powder, and even the right brand of cigarette papers to wrap bullets in, to get that extra 1/10 of a minute of angle, I would have found this highly interesting.
jimlongley wrote:Having reread the thread and posts related, I can see where this might be important, as someone said, shooting at coke cans at 1000 yards, but not really for us day to day handgun carriers. I can't imagine that a millimeter or two difference in placement would have a great deal of effect on a BG 25 feet away.
OTOH, when I was shooting long range and trying to find the best combination of metals, the right powder, and even the right brand of cigarette papers to wrap bullets in, to get that extra 1/10 of a minute of angle, I would have found this highly interesting.
I've never tried paper wrapping a bullet before, although it is intriguing. The last guy I heard of who did that was buried in the desert out past Terlingua. Still got the shovel.
jimlongley wrote:Having reread the thread and posts related, I can see where this might be important, as someone said, shooting at coke cans at 1000 yards, but not really for us day to day handgun carriers. I can't imagine that a millimeter or two difference in placement would have a great deal of effect on a BG 25 feet away.
OTOH, when I was shooting long range and trying to find the best combination of metals, the right powder, and even the right brand of cigarette papers to wrap bullets in, to get that extra 1/10 of a minute of angle, I would have found this highly interesting.
I've never tried paper wrapping a bullet before, although it is intriguing. The last guy I heard of who did that was buried in the desert out past Terlingua. Still got the shovel.
