Benchrest Bullet Design
Typically, benchrest bullets for under 200 yard shooting don't need to exaggerate the BC, and can gain more by using the lowest practical spin rate. Therefore, a 6-S ogive would be entirely practical. Many choose a 7-S custom ogive, or the 8-S ogive. A few choose the ULD (Ultra Low Drag) or VLD (Very Low Drag).
As a general
rule ULD design is not appropriate for medium and short range (100-300 yard)
target shooting simply because it forces the shooter to use a longer bullet than
necessary, which in turn requires a faster twist barrel, which in turn
exaggerates any jacket wall eccentricity. So why do it? Who cares about the BC,
if you are not shooting in a gale wind, at 100-300 yards? If you can read the
mirage and the wind flags like a high power shooter, then you can certainly take
advantage of the slower spin that stabilizes a normal weight 6-S bullet.
But the ULD design will help at 500-1000 yards (and of course, with 50 caliber benchrest at 1000-2000 yards, it will become a necessity as soon as enough other good shooters catch on). At some point, the bad effect of more spin balances the bad effect of wind drift on a lower BC bullet, and you choose the lesser of the two evils. This is no difference from other bullet design fields, where you are always choosing between two contradictory values and trying to balance their bad effects in order to get the most use from their good effects.
A 14 twist is the norm for 100/200 yard Benchrest rifles.
Standard Deviation (SD) is a statistic
that tells you how tightly all the various Velocities are clustered around the
mean in the string of data. SD is a presumption statistic from
chronograph data. Extreme Spread (ES) is a factual statistic from chronograph
To calculate standard deviation from Velocity strings, the variance is the average of the squared differences between Velocity points and the mean. Variance is tabulated in units squared. Standard deviation, being the square root of that quantity, therefore measures the spread of data about the mean, measured in units of Velocity.
In any data set, nearly all of the values will be nearer to the mean value, where the meaning of "close to" is specified by the standard deviation. We have the following weaker presumption bounds:
At least 50% of the values are within 1.41 standard deviations from the mean.
At least 75% of the values are within 2 standard deviations from the mean.
At least 89% of the values are within 3 standard deviations from the mean.
At least 94% of the values are within 4 standard deviations from the mean.
At least 96% of the values are within 5 standard deviations from the mean.
In general: at least (1 − 1/k2) × 100% of the values are within " k " standard deviations from the mean.
So.... a SD statistic from a 3-shot string is only a 89% probable statistic.
** A 5-shot string is only a 96% probable statistic.
*** A 7-shot string is only a 98% probable statistic.
ES totally depends on the caliber. Some calibers can yield
single digit and low teens, while others will yield the 20's and 30's.
As a rule of thumb, the smaller the capacity, the lower ES can be expected.
Many think SD is an over rated measure unless they come from at least 10-shot strings.
A SD 5-shot string is not a factual mathematical deviation equation.
Lowell Hottenstien’s BT
Email - firstname.lastname@example.org
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