Hi Jim,
Your story presents a good opportunity to demonstrate the power and simplicity
of the SI unit for plane angles, the radian. Since we're talking about small
angles, just divide the diameter of the grouping by the distance. For those
of us less talented than your one-eyed acquaintance, we might measure our
skills in milliradians (mrad). The calculation is,
Grouping in mm / distance in m = angle in mrad
For example,
25 mm / 25 m = 1 mrad
100 mm / 100 m = 1 mrad
50 mm / 25 m = 2 mrad
Your acquaintance was shooting
15 mm / 91.4 m = 0.16 mrad = 160 urad
Alternatively, if we know our goal and want to calculate the pattern size
angle in mrad * distance in meters = grouping in mm
For example,
0.8 mrad * 20 m = 16 mm
MORAL: the SI unit radians is BETTER (simpler, easier to use) than the non-SI
unit degrees for many applications. In customary units, it's just a lucky
coincidence that 1 arcminute is about 1 inch at 100 yards.
John
On Thursday 23 October 2003 07:39, Jim Elwell wrote:
> Just some random thoughts on angle and distance:
>
> I was at the local firing range the other day, talking with an old,
> one-eyed gentleman at the next bench. He was showing me his "half minute
> groups" fired at 100 yards (mighty respectable shooting for someone missing
> his "strong" eye).
>
> This got me thinking: any rifle enthusiast knows that "one minute" (of
> angle) is very close to 1" at 100 yards -- 1.0472" to be fairly precise.
> So, what would the equivalent in metric be?
>
> Well, 1 minute of angle is 29.0888 mm at 100 meters. Given the difficulty
> of accurately measuring the centers of groups of holes in paper (let alone,
> the "fish tales" issue), I think a good equivalent is that 1 minute of
> angle is 30 mm at 100 meters.
>
> Not that it matters: most the formal ranges (as opposed to open range or
> gravel pits) I've ever shot at would be very hard pressed to provide 100
> meter targets (about 8.5 meters longer than 100 yards). The mechanical
> devices that hold the targets cannot easily be moved, and you cannot have
> some people 8.5 meters behind others when shooting, for obvious safety
> reasons.
>
> So, given than most ranges in the US will allow only targets at 91.44
> meters, one minute of angle is 26.5988 millimeters. This does not round
> nicely, other than to 25 mm.
>
> Of course, the firearms industry is almost as bad as the printing industry
> in its use of different, archaic and (frequently) approximate measurements.
> Metric is creeping in faster than in printing, largely due to the US
> military metricating and wanting compatibility with our NATO allies. But
> new civilian items frequently are still solidly colloquial (e.g., 357 sig,
> 40 S&W).
>
>
> Jim Elwell, CAMS
> Electrical Engineer
> Industrial manufacturing manager
> Salt Lake City, Utah, USA
> www.qsicorp.com
