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Old January 21, 2010, 07:15 AM   #1
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Another about internal ballistics

I am trying to find a bullet for my AR-15. The gun has a 1 in 7 twist. What weight would you recommend? Is there a calculation one can do where you plug in the twist and you get the optimum bullet weight?
I have been thinking about a 50 grain or even lighter, but I would like to start with the best weight to begin with.
Thank you in advance for any help.
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Old January 21, 2010, 07:25 AM   #2
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Fast twist

My AR's are 1:9 and they love bullets lighter than 55 grains. Whenever I have loaded heavier I haven't been able to get good groups.
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Old January 21, 2010, 08:55 AM   #3
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1:7 is considered a very fast twist. Fast twist will stabilize heavier bullets (actually, longer bullets, but as we're talking about a single caliber and (I assume) jacketed lead, heavier means longer), but can "overstabilize" lighter bullets, resulting in decreased accuracy. In general, I think you're going to find better accuracy with bullets over 50 gr, and your 1:7 should stabilize up to just about any .223 bullet weight you're going to find. There's a great deal of discussion on this topic out there if you Google it.
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Old January 21, 2010, 01:33 PM   #4
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Barrel Twist Rate 1:14" Max Bullet Weight 55gr FB
1:12" best bullet 65gr FB
1:9" best bullet 73gr BT
1:8" bwst bullet 80gr BT
1:7" or 1:6.5" best bullet 90gr BT VLD
If your throat is cut long I would use the heaviest bullet I could find.
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Old January 21, 2010, 03:50 PM   #5
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As mentioned earlier, it is length that matters most, while weight matters, but not as much. So does velocity, but even less, except in the transonic range. If you have Excel or the free Open Office Suite's Calc program, you can download my stability and twist estimator file free here. If you have neither, a version of the same Don Miller calculations is available online among the JBM calculators. It's the last one when you scroll down the page. At the top of the page, click on "Lengths" to see their work-in-progress list of bullet lengths for using in the calculations.

Because length has more effect on required barrel twist than weight or velocity, same-weight bullets of different shapes don't have the same optimal twist. The 75 grain Berger VLD, 1.063" long, for example, needs about a 10% faster twist for that than the Sierra 77 grain MatchKing, which is blunt shaped for magazine feed, and is only 0.994" long.

The calculator's output is the gyroscopic stability factor, s. Its optimal value for accuracy is not quite agreed upon. Harold Vaughn thinks 1.4 is about right. Don Miller and a couple of other sources I've run into like 1.5. One source I saw recently liked 1.7. The truth is, it depends how well the bullet is made? Spinning too fast cuts into ballistic coefficient by increasing bullet yaw in flight which adds drag, but it also exaggerates any tiny imperfection in the symmetry of the bullet mass distribution around its spin axis, increasing its wobble and the helicity of its flight path around the trajectory line. A well-made bullet won't wobble as much when overspun as a poorly made one is likely to, so you can often get away with a higher s value with well-made match bullets. The military usually chooses a number closer to s=2.0 at Army standard meteorological conditions. That's so it will still be stable in more dense air, as when temperature, altitude, and humidity are reduced.

My Excel file version has instructions and an air pressure vs. altitude approximating tool (you have to check local weather to see how that varies at different times of year and under different weather conditions where you are planning to shoot). A second worksheet lets you work backward from groups fired in a gusting side wind to find what actual average s value your bullet has on its way to the target? Because rotational velocity slows less quickly than forward velocity, s tends to increase during the bullet's flight, except in the transonic velocity range. The attempt to find an optimal value is meant to apply to what you start with at the muzzle.

For most commonly available bullets, Old Grump's list will be right in the ballpark.
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