## Hit probability

Started by mman, May 14, 2011, 11:33:45 AM

#### mman

Cant is next interesting topic. I added cant dispersion classically (cos and sin components from total bullet drop) to my calculator and it seems that dispersion from +/-5 degree cant may be at 1000 meters more than typical dispersion from +/- 0.5 m/s wind reading error. This is huge dispersion and nearly all of it is horizontal according to this classical model. But is this model accurate enough? At least dispersion components from wind correction when canting should be noted also. But is there something more complicated behind this?

I found this from wep:

"I went out to my local shooting range and zeroed the rifle at 400 yards when ScopLevel #1 read level. I shot several groups to be sure of the zero. Next, I rotated the rifle until ScopLevel #2 read level. This produced a cant of 5°. I shot several groups at 400 yards with the canted rifle, and measured the effects each time.

The results? Surprising, to say the least. When canted, the bullets struck about 4 inches to one side. I expected some horizontal movement. But I did not expect the vertical movement to be as large as it was — the vertical impact averaged 3.75 inches low!

The .260 Remington is a cute little cartridge, but it is certainly no barnburner. I took out one of my 7mm Remington Magnums. This is one of the guns I shoot religiously every week at long range. I usually go to the range, place a target, and then try to guess how the environmental conditions will affect my bullet (I use a laser rangefinder, so the range is not a problem). On that particular day when I shot without any canting, my bullets hit 2 inches to the left and 3.5 inches high at 700 yards. I was more than pleased. I then shot with the rifle canted 5°. The results were astounding.

The bullets hit 6 inches to the right of the uncanted shots, but 8 inches low. The next week I broke out my .220 Swift, which at 700 yards has a trajectory similar to the 7mm. Its bullets hit 7 inches low when canted 5°. Both groups were under ½ moa. Future shooting confirmed my initial results.

All three rifles hit much lower than trigonometry calculations would have predicted. I cannot figure out why. The horizontal displacement was just about what I expected (my shooting was done in early morning under calm conditions), but the vertical displacement is difficult to explain. Certainly my shooting results are not statistically valid, but they convinced me that canting creates errors in both the horizontal and vertical directions." (http://www.riflescopelevel.com/cant_errors.html)

... I am in St Petersburg, RU and scarcely have time to analyse this case this weekend.  There is as far as I can see nothing special to the subject, just goniometry.  In the past I have used this graph for my calculations ...

Of course one might expect that a canted rifle interacts differently with a shooter/benchrest.

the right hand side sketches the view on top of the rifle

#### mman

Here it is again (flight time depended dispersion theory):