View Full Version : Specific velocity for a given rate of twist
November 15, 2009, 12:37 PM
A particular bullet/sabot manufacture has "recommendations" of bullet weights for my 1:28" twist Knight MK-85. However, just as important, it would seem there should also be a recommended velocity range that would be compatible with a certain bullet weight and rate of twist. I was going over the Greenhill formula and I couldn't figure out what I wanted.
Is there such chart anywhere? And if so, would I use the .40 cal bullet dimension or the entire .50 cal sabot? Thanks.
November 15, 2009, 07:47 PM
bullet length not velocity or weight is the determining factor, go with the manufacturers suggestion, they do not want unhappy customers and they have already done the research.
November 16, 2009, 03:05 AM
You are correct, the velocity is a primary determining factor in projectile stabilization.
The simplified Greenhill formula for lead/lead-core elongated projectiles will put you in the ballpark for twist rate but is not perfect. Use the projectile diameter, not the sabot diameter.
For velocities under 2,800 fps -
150 * (Diameter squared) / projectile length in inches = Twist in inches per turn
For velocities over 2,800 fps -
180 * (Diameter squared) / projectile length in inches = Twist in inches per turn
Spin-rate in RPM's is also a determining factor in projectile stabilization and is determined by the rate of twist in conjunction with the velocity. Since the twist rate is a fixed value, the only adjustment to spin-rate for a given projectile is via change in velocity.
Projectile spin rate is found by:
MV in fps * (720 / Twist rate in inches) = Spin-rate in RPM
MV in fps * (12/twist rate in inches) * 60 = projectile rpm
Any given projectile will have a given minimum & maximum RPM operational window. Projectiles having their total mass composed of less than 97% lead require use of the long Greenhill formula as well as taking into account specific gravity and projectile shape among other factors as well as those above.
Remember, always use the dimensions of the projectile and not those of the sabot or paper patch. When dealing with flat-base projectiles, you want to stay at the lower end of the RPM window in such a manner as to maintain sufficient RPM over the given linear travel distance of the projectile. RPM's must be increased on all projectiles exceeding the maximum allowable arc-angle within linear travel distance in conjunction with the relationship between projectile shape and length.
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