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Balllistics Program Question

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Unread 01-07-2003, 06:42 PM
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Re: Balllistics Program Question


It is much simpler than your trying to make it. If you know the actual bullet drop at your new range and relative air density, relative to the bore line, All you have to do is subtract your angular adjustment from the boreline that is already dialed on the scope under the 'guise' of being your old 100 yd. zero, at the old relative air density. What you get is the MOA move needed to hit the new target at the new relative air density, from the position of the original zero. I thought that was the original problem Blaine wanted to solve? If you want to get more exact, we need to throw in the Sight height stuff and your just about perfect.

[ 01-07-2003: Message edited by: S1 ]

[ 01-07-2003: Message edited by: S1 ]
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Unread 01-07-2003, 08:08 PM
Platinum Member
Join Date: May 2001
Posts: 2,369
Re: Balllistics Program Question


Help me out here, I'm apparently stuck on an incorrect assumption.

"(A-B)/c/1.047 = MOA correction from old zero to new target at current altitude

A = Actual bullet drop from borline at new Range and Altitude in inches

B = Actual bullet drop from boreline at 100 yd. old zero at old Altitude

C = range in yards divided by 100 "

Two scenarios:

1000 yards at 10500 ft ASL

200 yards at 10500 ft ASL

1000 yards first:

(A) Drop from boreline to 1000 yards = 347.1 inches

(B) Drop from boreline at 100 yards = 2.5 inches

(C) Range in yards divided by 100 (1,000/100) = 10

(347.1 - 2.5) = 344.6

344.6 / 10 = 34.46

34.46 / 1.047 = 32.913... (new 1000 yard come-up)

Now the 200 yards:

(A) Drop from boreline to 200 yards = 10.2 inches

(B) Drop from boreline at 100 yards = 2.5 inches

(C) Range in yards divided by 100 (200/100) = 2

(10.2 - 2.5) = 7.7

7.7 / 2 = 3.85

3.85 / 1.047 = 3.677... (new 200 yard come-up)

Now compare the "corrected" come-up to the original (non converted) come-up.

10500 ft ASL 1000 yard pre-converted = 347.1/10/1.047= 33.151...

10500 ft ASL 200 yard pre-converted = 10.2/2/1.047= 4.87...

The necessary MOA change for the 1000 yard shot is (33.151 - 32.913) = .238. I'll overshoot by .238 MOA (or about 2.5 inches)using the non-converted come-up.

The 200 yards MOA change is (4.87 - 3.67) = 1.2. I'll overshoot by 1.2 MOA (or about 2.5 inches) if I use the non-converted come-up.

It appears to me that all I'm removing is the first 100 yards flight time drop of 2.5 inches in either case. I know that I don't need 1.2 MOA correction at 200 yards at 10500 ASL and then only .23 MOA correction at 1000 yards.... What am I missing?
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Unread 01-07-2003, 08:36 PM
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Re: Balllistics Program Question

Your not missing anything, the formula is missing is the sight height correction, we need the actual sight height above the bore line to finish.
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Unread 01-08-2003, 08:39 AM
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Re: Balllistics Program Question

Holy Cow.. my head is starting to hurt.....
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Unread 01-08-2003, 08:53 AM
Platinum Member
Join Date: May 2001
Location: Pennsylvania
Posts: 1,757
Re: Balllistics Program Question

Hello All

This is just another reason we use the "spotter Shot" or two method "FIRST" when working over 1000 yards. NO drop chart is 100% correct when the conditions change from day to day after you have made the chart.

The spotter shot and a good spotter with bigeyes will eliminate the negatives of trajectory to the live animal.

Darryl Cassel
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Unread 01-08-2003, 09:19 AM
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Join Date: Dec 2002
Location: Asuncion, Paraguay
Posts: 39
Re: Balllistics Program Question

Lets have an aspirin together [img]images/icons/smile.gif[/img]...
If you are firing a 308 with a 175 SMK bullet @ 2680 fps, standard conditions and zero altitude, drop at 100yds is 2.53", and let's say sight height is 1.6" (this example is with the Sierra Infinity program, but exact numbers are not important).
In order for line of sight to coincide with the bullet trajectory at 100yds, you have to slope down your line of sight (or the erector tube inside the scope) to compensate for the sight height and the bullet drop at that distance. This compensation is 2.53+1.6 = 4.13" in 100yds.
Using a little trigonometry, we find that the proyection of the bore line crosses the line of sight at 38.74yds, and this means that the bore is pointing high 4.13*9.6126 = 39.7" at 1000yds.
If you substract this value from your drop at 1000yds (424.1") you have the bullet path relative to the line of sight at this point: 424.1-39.7 = 384.4". So, our sight correction for 1000yds would be 36.7 MOA.

Corrections to bullet drop at 100yds for air temperature, pressure and altitude are too small (this means the bullet keeps the same 100yds zero), but this same round fired at 20F could easily be going 100 fps slower (2580 fps), so the bullet drop is now 2.72". Using the above calculations we see this causes a 2.72*2.53 = 0.19" variation in our 100yds zero, and a 0.19*9.61 = 1.83" variation at 1000yds, this is a 0.17 MOA error at 1000yds, IMHO this is "lost in the noise", even with this extreme environmental change.

Edited for correctness...

[ 01-08-2003: Message edited by: TiroFijo ]
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Unread 01-08-2003, 09:58 AM
CAM CAM is offline
Silver Member
Join Date: Jan 2002
Location: Utah
Posts: 385
Re: Balllistics Program Question

Brent and Dave

Thanks for the answers and the data points.

The reason I ask about MV is because its affects on down range look to be more than most other Items.

MV is the result of internal ballistics and is the most important start for external ballistics.

I have heard guys keeping there bullets in a cooler to control powder temp and Elevation should not matter untill external ballistics.

I will do some averaging and just use one MV

I'll take two aspirin and reread the formula!

Thanks again

P.S. Dave I think you are using 100 and 1000 data when you should only compare old 100 and new 100 then you could compare old 1000 to new 1000 ? NOT SURE

[ 01-08-2003: Message edited by: Cam ]
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