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Need some help extrapolating my drop data.

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Unread 03-16-2010, 08:12 PM
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Join Date: Sep 2009
Location: SW MT or noVA
Posts: 986
Need some help extrapolating my drop data.


I have used exbal to work out some drop data, but i cant get the computed numbers to match my real world drops. Can someone who maybe has a better knowledge of ballistics chime in and/or use a more sophisticated program and edits BC? My velocity is right around 2844fps. I am shooting a 208gr A-max. published bc is .648 or .233 G7 according to Bryan's book (this is off memory dont take my word for it).

Input parameters:
Temp: 55 degrees F
Pressure: 29.7
Altitude: 32ft over sea level
humidity: 78%
Velocity: 2844fps (measured with chrony, average over 5 shots, i know it's not much.)
Sight height: 1.64"
Zero: 100 yards

Real world drops:
100yrds -- 0 moa
200yrds -- 2 moa
250yrds -- 2.75 moa
300yrds -- 3.5 moa <-- rock solid
370yrds -- 5.75 moa
400yrds -- 6 moa <-- rock solid

these are the drops I was using most were derived from reactive targets 3.5" in diameter so allow for a little variance. 300 and 400 yard drops were on steel and I adjsted till they hit perfect center line.

I am thinking about just graphing my real world drops in excel or such and then extrapolating. Will see, i will measure drops in inches from 100 to 400 in 50yrd intervals next saturday.

I am going to texas on a boar hunt next tuesday and am trying to get this stuff ready or ill waste a day of my hunt working up drop charts, instead of just a few hours confirming it.


Last edited by Oliveralan; 03-17-2010 at 05:57 PM.
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Unread 03-16-2010, 10:38 PM
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Join Date: Jan 2004
Location: Blackfoot, Idaho
Posts: 8,876
Re: Need some help extrapolating my drop data.


Me thinks something is goofy........

Here's what I came up with entering your real world drops:

-------------------------My --------------Exbal.
-------------------------Exbal----------- Adj.
------------------------ Run----Diff------Drops
100yrds -- 0 moa ----0--------0--------0
200yrds -- 2 moa-----1.25----+.75----1
250yrds -- 2.75 moa--2.25----+.5-----2
300yrds -- 3.5 moa---3.50-----0-------3
370yrds -- 5.75 moa--5.25-----+.5----4.5
400yrds -- 6 moa------6.0------0-------5.25

Reaffirmation of the 200/250 and 370 yard data may be needed????

Adjustments (I've only done this for real twice for my 270 AM and 338 RUM thus a grain of salt is appropriate) But both are spot on the 338 to 900 and the AM to 1400.

Here we go.

Lets either drop the 200 yd data as the 250 and 300 agree. However the the data at 300 and 400 conflicts w/the 600 drop.

Now we're stuck.......Exbal wants to know the sight adjustment made for each distance beyond the zero distance. I'm gonna assume that hold over was used as I don't know of many scopes w/mil dots and mil turrets. If that is the case the inconsistencies in the drops may be due to interpolation between mil dots, especially if the reticle is a mil between the dots with no hash marks for 1/2 mil.

You may have held zero for all shots and logged the drops below the Point of Aim. Thus there are no adjustment to enter into Exbal......... (Just a Note: when doing this you'll find that your drops are off by the cosine of gravity. I attended a lecture about that one time. Have no idea of how much it is but for me I'm confident its negligible. )

I'm confident that if these data are entered into excel it will be tough to get R^2 to 1 or at least 5 - 9s. Also I think that if excel were used and the curve adjusted, by dragging the inconsistent points you'll come pretty close to Exbal but with either reaffirmation of the drops will be necessary.

I entered the Exbal run drops as "Sight Adj Used" then entered the differences in drops at 200, 250and 370. I also included 300 with a +/- MOA of 0.0.

Exbal said the average MOA adjustment was -0.43 MOA and calculated the velocity to be 3000. (The velocity adjustment is the way that Exbal compensates rather than adjust the BC. I think......)

Hopefully someone w/more experience w/exbal will chime in. I've been doing things with excel for years with pretty great success but like code such as exbal much better (except for the velocity adjustment thing.)


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Unread 03-16-2010, 10:45 PM
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Location: The rifle range, or archery range or behind the computer in Alaska
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Re: Need some help extrapolating my drop data.

You have illustrated exactly why I start drop tests from a 300 yard zero instead of a 100 yard zero. Doing it the way you are doing it will drive you nuts, as you are finding out. There is too much going on under 300 yards and it is hard to get a true 100 yard zero to begin with. If you are .3" off center you might call that a perfect 100 yard zero when in fact, it really is not good enough. Now if you are .3-.5" off at 300 yards, the effects of that are very minimal. You can still test at 100-300 yards so you know where your dial is, however, for testing purposes, starting at 300 yards where things have settled down helps tremedously. Zero at 300 and shoot 350, 400, 450, 500, 550 and 600 yards and then try and match those numbers with your program. I would be willing to bet you will find a MUCH closer match. If it still doesnt match well, then change your BC in the program until it does match. I think that you will find that somewhere between .640-.660 will do the trick. Published is .648 but I have seen this vary with velocity and other factors. .648 is darn close though for a factory published value.

From there, your sub 300 yard values are very close even if they appear to be off a bit at times. It helps if you fire all those loads over the chrony so you can decern what is going on and get a better average.

Aside from that, it does seem that 2844 is a bit on the fast side for the 208 in a 308???????? Is your chrony accurate?
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
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Unread 03-16-2010, 10:54 PM
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Join Date: Sep 2009
Location: SW MT or noVA
Posts: 986
Re: Need some help extrapolating my drop data.

Roy, thanks for the help.

I was getting the exact same thing from my exbal. I could not get the 200yrd drop to fit into the equation. I will do the whole thing again Saturday and measure drops in inches at all the ranges. My scope is 1/4moa and I know the 300 and 400 is rock solid because I was putting shot after shot dead centered on the steel plates. The others I was shooting 3.5" dia clay pigeons so it could be off by a slight margin. I did usual inspect the clay disks to seE where bullets hit. However I shot the 200yard drop on paper so I'm quite sure it's correct. The only explanation is a different BC, or ofcourse I did something stupid (just as likely as a different bc). Of the curve cannot be achieved with changing velocity, the BC mus be off, assuming my drops are correct.

Well exbal is dead on for 300 and 400, the two I'm 100% sure are correct. The other drops aren't off by a lot so could just be error on my part.
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Unread 03-16-2010, 11:00 PM
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Join Date: Nov 2008
Location: SW Montana
Posts: 5,804
Re: Need some help extrapolating my drop data.

For site height measure with a caliper, measure the tube of the scope and the diameter of the action or barrel and them together and divide by 2 then add to that the measurement from the bottom of the scope to the top of the spot you measured the barrel or action.
I had to adjust the scope height to 2.0 to come up with decent numbers that were the same a Roy's but with Loadbase 3.0. Something is a little wack, but in that short of a distance it can be hard to tell if it is shooter, gun, load or ballistics program.
Also have you proven what your true sight adjustment is in, is it actually moving in the increments you think.
"Pain is weakness leaving your body"
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Unread 03-16-2010, 11:05 PM
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Join Date: Sep 2009
Location: SW MT or noVA
Posts: 986
Re: Need some help extrapolating my drop data.


was hoping you would post! Thanks that's some real good info on zeros. Never really thought of that. The problem is I can only shoot the 400 on my property, maybe 450 if I take the chainsaw to a bunch of trees.

You took the time to write such a long informative post but unfortinualy I can't really answer with an equaly information ladden on. What you said makes perfect sense and now I feel stupid for not having done it

I agree, 2844 is smokin! But, I fired 25shots through the chrony during load work up (only 5 at the charge I'm using now however) trebark was there and witnessed all this. He also shot his .280 rem through the same chrony just before and was getting the reading he expected (3100fps with a 140 nosler BT) so I don't think the chorny is off. The data I got from inputting my 300 and 400 yard drops gave me right around the same thing, and Roy's calculations are close enough to my real world drops that I wouldn't have noticed the difference shooting the clay disks. Therefore I'm reasonably sure my rifle is shooting them at 2800fps+. I am really heating up the barrel however... 3-5 shots and my barrel is cooking. And it's 1.25" in dia.
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Unread 03-16-2010, 11:13 PM
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Join Date: Sep 2009
Location: SW MT or noVA
Posts: 986
Re: Need some help extrapolating my drop data.

I haven't tested if my scope is true .25moa but at these short distances I'm not sure if I could tell .25" vs .25moa a friend has lent me a 3.5-15x50 NXS so that will be true .25moa. I may be going to quantico (local 1k range) and shooting Saturday. If everything is right, It should take 28-32moa for 1k. I will zero at 300yards with the nightforce and gather some data at intermediate ranges as well.

I will measure the sight height with a caliper tomorrow afternoon. My scope is canted due to the 20moa base. At what point do I measure the sightline over the boreline? I have the 20moa taken out by dialing my scope down to a 100yrd zero but the scope is still not parralel to the bore. Do I measure at the front of the tube before it widens to the obj width or...?
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