J'avais trouvé la question de lt55009 intéressante, je l'ai posé à Brian Litz et voici sa réponse:
From: Bryan Litz [mailto:
[email protected]]
Sent: Thursday, November 10, 2011 8:08 AM
To: Serge
Subject: RE: How to mesure the scope height ?
Serge,
I agree that your described method of measuring scope height is valid. Effectively, it finds the distance from the bore centerline to the line of sight (LOS). There are alternate ways to do this; for example, you can measure from the center of the windage knob to the center of the bolt, estimating the center of each. Also, you can measure from the center of the scope eyepiece down to the center of the bolt shroud. The point is to find reference points that will align with the LOS and bore line. Granted, I think your method is probably the most accurate since it relies on direct measurements rather than estimations, but to be honest, this is not a measurement that requires great accuracy. If you get it within +/- 1/8" that's plenty close enough.
To illustrate why it's not terribly important, consider that when you're shooting long range and have 20, 30, or 40 MOA dialed on the scope, this effectively puts the LOS closer to the bore line at the muzzle, which is where the ballistics program is applying the 'sight height' variable. The program will apply the constant number you provide even though in reality the height changes as you dial the scope (the ballistics program considers the muzzle to be distance = zero). So because there is variation in this number that can be on the order of 0.100" to 0.300" as you change the scopes elevation setting, it doesn't make sense to measure the exact sight height to the nearest 0.001" at the scope bell. In addition, the trajectory prediction simply is not highly affected by small changes in this number anyway.
Regarding probability contour; the probability contour comes from the famous 'bell curve' of normally distributed data in which 67% of the data will lie within +/- 1 standard deviations (SD's) of the average, 95% of the data will lie within +/- 2 SD's of the average, and 99% of the data will lie within +/- 3 SD's of the average. So for example, if your horizontal dispersion is driven by uncertainty in windage, and you quantify that uncertainty by saying that the shooter judges the wind within +/- 3 mph, 95% of the time, then the resulting horizontal dispersion represents +/- 2 SD's, and is the 95% line in the probability contour. Another line with 1/2 the radius (horizontally) represents +/- 1 SD, and is the 67% line in the probability contor. Likewise, a line that is +/- 3 SD's is the 99% line in the probability contour. To do it in the vertical plane, consider the effects of muzzle velocity variation. If your load has an average muzzle velocity of 3000 fps with an SD of 10 fps, then 67% of your shots will be within +/- 10 fps (1 SD) of the average, from 2990 to 3010 fps. 95% of your shots will be within +/- 2 SD's (2980 to 3020 fps), and 99% will be within 3 SD's (2970 to 3030). The vertical dispersion associated with these velocity windows defines the top and bottom of your probability contours.
I hope the above makes sense, it is a somewhat complicated subject.
Take care,
-Bryan
Bryan Litz
Ballistician
Applied Ballistics, LLC
15071 Hanna Ave
Cedar Springs, MI 49319
Office: (616) 263-9284
Cell: (937) 776-1209
http://www.appliedballisticsllc.com/ From: Serge
Sent: Friday, November 04, 2011 9:17 PM
To:
[email protected]Subject: How to mesure the scope height ?
Hi Brian,
I've posted on a forum (in french), a topic about ballistic, and add pictures how to mesure the scope height, from my own understanding. One member is not so confident about my measuring technique and now, I have myself doubts about it. I measure the outside diameter of the scope and divide by 2, same for the barrel, and I measure the space between the scope and the barrel with a feeler gauge. You can see the pictures of "how I do it" with this link
viewtopic.php?f=9&t=22850 Is there a better way to do it ?
In your first edition of your book, at chapter 15, Hit Probability For Hunting.
I tried to figure out how you determine the % in the proabilty contour.
I have no problems with the RSS when I transpose this to other values but I'm not able to determine how to calculate the probability contour.
Regards.
Serge
Rimouski, Qc
Canada
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