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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/17/2010 at 21:53
Corndawg View Drop Down
Optics Apprentice
Optics Apprentice

Joined: March/04/2010
Location: Bama
Status: Offline
Points: 74
I purchased a Vortex Viper 6-24x50 w/target dot that was on clearance here @ SWFA.
It's mounted on a .257 STW w/TPS rings and Warne 20moa rail.
I'm considering a Vortex or Kenton turret for it.
Now fps and bc are pretty easy to figure out, but elevation is looking tricky.
Wikipedia says my area is 194'. (3) other websites list the elevation @ 190', 162', and the lowest has it @ 121'.
What do I do? Split the difference?
How much of a differnce will a max spread of 73' make, or am I splitting hairs here?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/18/2010 at 00:44
8shots View Drop Down
Optics Jedi Knight
Optics Jedi Knight
Lord Of The Flies

Joined: March/14/2007
Location: South Africa
Status: Offline
Points: 6040
Not a lot, to nothing.
What is more important is to know is the air pressure on the day that you are shooting. This varies from day to day of course, depending on the weather.
If you are shooting inside 300 yds then most of this do not matter to much.
BJM gives at 300yds: 200ft = 12,5 inches and at 4000 ft = 12,1 inches
At 800 yds things change a bit
200 ft = 182 inches and at 4000 ft = 171 inches.
temp plays a role.
10 deg F at 800 yds = 197 inches
90 deg F at 800 yds = 182

Edited by 8shots - November/18/2010 at 07:36
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/18/2010 at 06:34
jonoMT View Drop Down
Optics Master Extraordinaire
Optics Master Extraordinaire

Joined: November/13/2008
Location: Montana
Status: Offline
Points: 4833
Without a doubt, barometric pressure is the variable that has the most effect on trajectory, followed by temperature and altitude (although large variations in either will matter). Humidity isn't much of a factor. I think for most shooting out to 500 yards you could add calibrated turrets and have a pretty decent setup. But the problem with any BDC system (whether reticle or turret-based) is that it is only perfect for one load and one set of conditions. That's why I say it will be decent inside 500 yards.

If I wanted to take a 600 yard shot on a 30 deg. slope at an altitude 1500 feet above where I zeroed, I'd be more comfortable with a generic set of turrets (e.g. mils) and a handheld ballistics calculator. But to address this particular question, I'd say figure out the averages for all your variables - pressure, altitude, temp and humidity - and make sure you have very accurate numbers for BC and muzzle velocity. Then BEFORE getting the turrets made, go out and shoot at 100 yard increments out as far as you can to verify the numbers you get in JBM. Ideally, do this on a day where the conditions match those averages.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: November/18/2010 at 08:10
Ernie Bishop View Drop Down
Optics Journeyman
Optics Journeyman

Joined: January/26/2010
Location: Gillette, WY
Status: Offline
Points: 667
Very few ever shoot a 30 degree slope.
It sure can, and does happen though.
One has to have the means to measure slope in the field, and then be able to know what it translates into-Sadly, not a lot of guys carry that with them in the field.

Other than that, you will not a see a lot of difference. 
Just for giggles--I would "guess" 1/4 MOA less drop at 600 yards from the conditions mentioned above.
As mentioned, verifying your drops is a must before I would get a turret made up-No way around it, if you want it to be reliable.

You never did mention what your max range would be???
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