Need a drop chart

tlk

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Apr 11, 2008
Messages
238
Is there a website, freeware, etc where I can use my load data to get a reliable drop chart? MOA, not Mils.

Thanks.
 

tlk

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Apr 11, 2008
Messages
238
OK, I am a little confused.

I used the JBM site and what I am needing is a chart that tells me how many MOA I need to dial in for elevation. Is the Drop(MOA) column that information?

What I was looking for was something like: 300 yards, +2.5 MOA.

Lot of info there on the JBM application.
 

300rum

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Jul 27, 2006
Messages
564
Location
AB, Canada
OK, I am a little confused.

I used the JBM site and what I am needing is a chart that tells me how many MOA I need to dial in for elevation. Is the Drop(MOA) column that information?

YES. YOU ARE RIGHT ON..
If you change the value to MIL you will get the dial in MIL.

Just as info, if you do not know what to do with JBM at a specific field input just click on the "?" attached to this particularly field. You will jump to a help page.
 

JUDD

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Jun 25, 2007
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946
Location
BLOOMSBURG, PA
If you have Excel on your computer I have one you can use. Pm me your email address and I'll send it to you.

I like it cause I can take my laptop out and use it and don't have to worry about having internet conncetion to get to JBM.
 

tlk

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Apr 11, 2008
Messages
238
Thanks all.

When I run JBM and Jacksonrifles.com against each other I get 1.2 MOA difference at 600 yards. I thought I would see differences, but that seems extreme. I was careful to enter the exact data in both, including elevation, wind, pressure, etc.

What gives? Since it SHOULD all be science by now, I would think that any error would be rounding and minimal.
 

bigngreen

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Nov 24, 2008
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8,197
Location
SW Montana
Which JBM chart are you using? I have found the small drop charts are off and need tweaking but the main trajectory cal is good and will match with other quality ballistic calculators.
If you post up your data we could have a look at it. I'm running Loadbase 3.0 and out to about 1000yrds JBM is fairly close if I have everything spot on, maybe give you some more perspective.
 

tlk

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Apr 11, 2008
Messages
238
JBM:

Manufacturer: Sierra
Description: Spitzer Boattail (GameKing)
Caliber: 0.308 in
Weight: 165.0 gr
Ballistic Coefficients: 0.419 [0-1600], 0.409 [1600-2400], 0.404 [2400-5000]
G1 Muzzle Velocity: 2983.0 ft/s
Distance to Chronograph: 10.0 ft
Sight Height: 1.50 in
Sight Offset: 0.00 in
Zero Height: 0.00 in
Zero Offset: 0.00 in
Windage: 0.000 MOA
Elevation: 0.000 MOA
Line Of Sight Angle: 0.0 deg
Cant Angle: 0.0 deg
Wind Speed: 10.0 mph
Wind Angle: 90.0 deg
Target Speed: 0.0 mph
Target Angle: 90.0 deg
Temperature: 70.0 °F
Pressure: 29.92 in Hg
Humidity: 75.0 %
Altitude: 1047.0 ft
Vital Zone Radius: 2.0 in
Std. Atmosphere at Altitude: No
Pressure is Corrected: Yes
Zero at Max. Point Blank Range: No
Target Relative Drops: Yes
Mark Sound Barrier Crossing: No
Include Extra Rows: No
Column 1 Units: 1.00 in
Column 2 Units: 1.00 MOA
Round Output to Whole Numbers: No


Jacksonrifles.com:

Sierra 165 GKBullet description
2983 Muzzle speed (fps)
165 Bullet wt (gn)
0.474 BC1
00 Special range (yd)
0 Start range (yd)
50 Increment (yd)
0.00 Impact Ht. (in)
100 at Zero range (yd)
10 Wind speed (mph)
3.0 Wind dir. (o'clock)
20 Temp. (Celcius)
1047 Altitude (feet)
1000 Pressure (mbar)
1.50 Scope Ht. (in)
1.05 in./MOA @ 100 yds
0.50 ******. Coeff. rate
1300 Break Velocity (fps)

Calculated data & constants:
0.502 Adjusted BC
4269 Retardation coeff.
4521 Adj. ******. Coeff.
1000 Standard pressure
246 Mayewski constant
68 Temp. (Faht.)
2.04 Drop at Zero

I found part of the issue - Jacksonrifles.com has the temp in celcius. Have to jack it down to get it to where you want it in Farenheit (sp?). Still have .7 MOA delta at 600, though.


Thanks for your help - it is much appreciated.
 
Last edited:

bigngreen

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Nov 24, 2008
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8,197
Location
SW Montana
It looks like you may have two different BC's. I plugged it into Loadbase 3.0 and it agreed with JBM, I used a single BC of .409, if I lay a track along side with a BC of .474 it put a .7 MOA difference.

 

tlk

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Joined
Apr 11, 2008
Messages
238
I Used JBM and Big Game Info for shots at 400 yesterday. Everything worked out.

BUT - I have question: Bigngreen, why did you use the BC of .409? Doumentation in Point Blank has a BC of .474, which is what I used. When I look at the JBM info I posted below I see that there are three different BC's (based on velocity, I assume, please correct if wrong). Based on what you found, I am wondering if this is where the difference I am seeing can be traced to.

Is JBM calculating based on different BC's at different bullet speeds during flight?

Thanks.
 

bigngreen

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Nov 24, 2008
Messages
8,197
Location
SW Montana
I used .409 because it is middle of the road. Sierra uses three BC's but I did not know that JBM would use three but it kinda looks that way. JBM also lists Litz's BC for that bullet and I have found that to be very accurate but I don't know what it is, I'm going to get his book on order ASAP.
If I used .409 in Loadbase and the library file in JBM they were within a 1/10th at 600yrds. With Loadbase I can lay out multiple tracks next to each other so I used .409 for one and another at .474 and they where .7 apart in the same manner, it sounds like you had.
I started this thread, feel free to take a run at it with JBM it is good practice and for fun.
http://www.longrangehunting.com/forums/f19/warm-up-your-ballistics-calculator-52033/
 

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