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 Posted: Sat Aug 13, 2011 1:41 pm |
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| rsterne |
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| Location: Coalmont, BC |
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I found a twist rate calculator that actually has SUBSONIC values.... It calculates the minimum (slowest) twist for a stability of 1.5.... You probably already know that <1.0 is unstable, 1.3 is what the "benchrest guys" aim for, and the Military use 1.5-2.0, so 1.5 at sea level and normal temperatures is a practical minimum to use....
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| This program is basically a slick version of Robert McCoy's "McGyro" DOS program, written in the late 1980's when he worked for the U.S. Army Ballistic Research Laboratory at the Aberdeen Proving Grounds. It was later improved by William Davis Jr. of Tioga Engineering and the claim was an accuracy of 5% for super and subsonic velocities, and 10% for trans-sonic velocities. |
http://www.border-barrels.com/barrel_twist.htm
If you put in some values you will note that it produces a graph which has a discontinuity around the speed of sound.... Below that, MORE twist is required, and not just a little bit.... Something to bookmark and play around with for sure.... I'd recommend that you use a lower Specific Gravity than pure lead to allow for the reduced gyroscopic effect of a diablo (waisted) pellet with a hollow base because it will have a smaller rotational moment of inertia than a solid bullet with parallel sides and a flat base.... ie from a purely rotational point of view it would need a higher twist.... However, since pellets are drag stabilized maybe we can ignore that?....
It should provide extremely good values for solid cast bullets as typically used in Big Bores, however....
Bob |
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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal: Too many to count! |
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| Posted: Sat Aug 13, 2011 7:38 pm |
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| Gippeto |
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Some random and slightly related thoughts.
Ever wonder WHY military rifles (at least since the switch to <.30cal) use a higher twist rate than required? Believe the effect is called trailing, but it HAS been a few years....the over spun bullet maintains the nose high attitude it had when leaving the bore...all the way to impact.
Travelling in a downward arc but nose high...the effect upon impact is a buzz saw. Nasty stuff.
Yeah...another theory...not the kind of thing they'd publicize.
Might want to look into subsonic bullet design as well...might be room for improvement over what's currently being "pushed for profit". 
http://members.shaw.ca/cronhelm/SubsonicBullet.html
Al |
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| Posted: Sat Aug 13, 2011 8:11 pm |
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| rsterne |
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| Joined: 15 Jan 2008 |
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| Location: Coalmont, BC |
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Good article, I've seen it before....
Bob |
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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal: Too many to count! |
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| Posted: Sat Aug 13, 2011 9:30 pm |
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| rsterne |
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| Joined: 15 Jan 2008 |
| Posts: 2998 |
| Location: Coalmont, BC |
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Interesting how the stability drops drastically in the transonic region.... Here is an example of a bullet that is normal stability when supersonic, but goes unstable as it drops through the speed of sound.... However, at under 1000 fps it is stable again....
Minimum stability is 1.0, and that example drops to about 0.93 at 1100 fps.... I used a .22 cal round nose bullet 0.5" long and an 18" twist to get that graph....
Bob |
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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal: Too many to count! |
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