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Ballistics Calculator Problems!

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Unread 02-16-2010, 10:34 PM
Junior Member
Join Date: Oct 2009
Posts: 26
Ballistics Calculator Problems!

I am having some problems with ballistics calculators. I mean, a lot of problems. A little history first.

I am shooting a Savage 10FCP in .308. I have her zeroed at 100 yrds. I don't have a chrony but I have a rough idea of what my MV is. I have BC's supplied by Sierra's website for my bullets. 180gr SMK's. So the data that I input into the calculator(s) is MV, BC, bullet weight, altitude, temp, zero range, sight height, and air pressure. And the calculator(s) crank out a range card. The range card is fairly accurate out to about 500 yrds. At least they are accurate enough to get on steel silhouettes to 500 plus. But the sight in range I have has a steel deer at 600 and the drops put me WAY over the deer. And as I get out to 1000 I am WAY over the target with drops supplied.

Now, I know how many clicks I need to use in the real world to get on target at those ranges. Or, at least, I know roughly how many clicks I need to use to get on target at those ranges.
100 yrds = 0 clicks
300 yrds = 22 clicks
400 yrds = 30 clicks
600 yrds = 47 clicks

Now my calculator, with correct info is giving me clicks of 0, 20, 33 and 63 respectively. And that was closer than I was a bit ago. (explained below)

On a side note, I am a firefighter and am now going through a Wildland Fire Weather course where we were talking about air pressure's. So I used that information to my advantage as a shooter. In short, air pressure will push up 30" of mercury at sea level. You will lose 1" per thousand feet of rise. So at my altitude I will lose 6" of mercury. That puts me at 24". So if I watch the weather and it says barometric pressure is 29.92" I will input 23.92" in my ballistics calculator.

Now, that correction puts me closer to the real clicks that I am finding on the range. I thought that was weird since my calculator asks me for both pressure and altitude. I could see a couple clicks here or there since I don't know the actual MV of the bullet but 16 clicks difference at 600 yrds is a lot. And if I change the MV to WAY over what I know it really is I can get my 600 yrd drops right but then my closer drops are way off.

What am I missing or are ballistics calculators really that inaccurate?
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Unread 02-16-2010, 11:24 PM
Silver Member
Join Date: Jun 2006
Location: buffalo, OK
Posts: 400
Re: Ballistics Calculator Problems!

I run exbal on computer and ballistic fte on my iphone, they both are very accurate if the info is put in accurate.

What i would do in your case if i didnt know my mv or true bc is set up targets at 200, 600, 800 or 1000. Shoot each one and write down your moa readings for each of those distances and your atmo conditions. Then you can play with your mv and bc to make the ballistic software match your data for those atmo conditions. Once you get that to match it should be correct when you input condition changes for other places. You might need to play with your bc more than your mv if your are getting close one way and farther the other. I know my bc calculations come out pretty far off if i dont put a bullet out around 800yrds to figure in.

Without knowing mv and true bc ballistics can be pretty tough to get to match, but with real range data dont be afraid to play with your bc or mv to get your ballistics to match

Also if you input true pressure do not input altitude, leave it zero

I did mine that way for years before i got a chrony and all the other little toys. I still shoot real range data and make cards even though i have ballistic computers.
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Unread 02-16-2010, 11:31 PM
Junior Member
Join Date: Oct 2009
Posts: 26
Re: Ballistics Calculator Problems!

Thanks, I will give it a shot.... no pun intended.
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Unread 02-16-2010, 11:50 PM
Official LRH Sponsor
Join Date: Mar 2007
Posts: 634
Re: Ballistics Calculator Problems!

It's essential to know the true value of your scope clicks. The program assumes that your clicks are precisely 1/4 MOA. The truth is many scopes can have up to +/- 10% or more error when you actually adjust the scopes. In other words, when you move your scope 60 clicks, it may actually be moving the crosshairs 54 or 66 clicks (more or less).

If you haven't done it yet, you can set up a 'tall target' at 100 yards. Shoot a group at your 100 yard zero then move the scope up 60 clicks and see if it moves the POI precisely 20 MOA or if it's more or less than that.

It may be possible that the program is giving accurate output but your scope isn't moving as it should.

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Unread 02-17-2010, 12:50 AM
Platinum Member
Join Date: Nov 2008
Location: SW Montana
Posts: 5,804
Re: Ballistics Calculator Problems!

I punched some rough numbers into JBM and I came close to what you had. Are you sure that the target is 600 yrds, it matches to well at your other ranges, but for 600 yrd it is really jacked up all of the sudden when the others where close, it's close to what the 500yrd should be. 1 click = 1 inch ???
Ballistic programs are generally as accurate as your info, when you see something that is way off there is an incorrect input and you just need to hunt it down.
"Pain is weakness leaving your body"
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Unread 02-17-2010, 08:49 AM
Gold Member
Join Date: Feb 2008
Location: Sonoma County, CA
Posts: 827
Re: Ballistics Calculator Problems!

Good advice so far. Another thing to consider is your muzzle velocity and how accurate is it. When you say your drops are close enough to put you on steel up to 500 yds., that depends on how big that piece of steel is. Just an example, if you are shooting at a 20" piece of steel and hitting it high, that will only compound at longer ranges. If your ballistic program allows you to do a trajectory validation, that may help.

You could post all of the information for your load, and some of us can see what we come up with for drops. What program are you using?
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Unread 02-17-2010, 09:16 AM
Junior Member
Join Date: Oct 2009
Posts: 26
Re: Ballistics Calculator Problems!

Good info so far guys. BSL, I will try that with the scope. I am using a program or calculator that is in an excel spreadsheed that I downloaded from here.

Free Ballistic Calculator (FREE Ballistic calculator)

FBC-v3.xls is the file name.

I have the option of setting the click value and I also have the option of setting the MOA value against an inch. Apparently not all scopes agree on how many inches an MOA is. Who'da thunk it.

I have also used JBM a lot as well as some of the calculators that some of the bullet makers have on their sites. And have come up with similar results.

bigngreen, do I know that the target in question is really at 600 hundred? Well, that's what it says on the sign behind the shooting bench! LOL. The site in range is 200 long, behind that is a deer, the behind that is an antelope and bear and behind that is another deer. The sign at the range says deer 300 bear and antelope at 400 and mule deer at 600. Have I actually measured it or used a range finder in it? NO!

I suppose that could be a problem as well huh?

Moman, I am having problems with the steel that was listed above and they are life sized steel. The steel that I don't seem to be having a problem with is regulation sized steel silhouettes. Pigs at 300 meters, turkeys at 400 meters and rams at 500 meters.

Quick load data I am shooting 180gr HPBT SMK's using Federal brass, CCI 200 large rifle primers. And somewhere in the middle I put 41 gr's of Ramshot TAC powder. Ramshot powder lists that powder with 180gr HDY SST bullet with a low charge of 38.5 gr at 2325 fps and a top load of 42.8 gr's at 2585 fps. That is with Ramshots rilfe with a 1:12 twist and 24" barrel. Mine is 1:10 twist and 21" barrel.

I have to go to work today and will be at work for the next day in a half. I might get on tonight as time permits.

Thanks for all the help so far.
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