Help with a drop table.....

[ColinR]

I’m hoping some of you with experience building a drop table can help me out with some questions.

I’m done with load development on 3 rifles so today I shot them out to 300 yards. I know not long range but the longest I had access to right now. All 3 rifles grouped very well, all maintaining less than 1 Moa at 300.

Here are my findings. Muzzle velocity’s were taken with magnetospeed v3.

308. Muzzle velocity 2745 shooting 165 grain Nosler ballistic tip
Zeroed at 100
200. 6 clicks or 1.5 moa drop
300. 16 clicks or 4 moa of drop


6.5 saum muzzle velocity of 3090 shooting 130 grain Sierra tipped game king
Zeroed at 100
200 4 clicks or 1 moa
300 12 clicks or 3 moa


28 nosler muzzle velocity of 3025 shooting 175 grain Nosler Accubond long range
Zeroed at 100
200 4 clicks or 1 moa
300 10 clicks or 2.5 moa

Now here is where my question comes in. When I put my bullet ballistic coefficient and muzzle velocity into the drop calculator my actual findings don’t match the drops listed on the calculator. This is using both the Nikon spot on app and JBMcalculations. In order to make my actual findings match the drops on the calculator I have to increase my muzzle velocity.....quite a bit. Is that the proper way to do it? Changing the velocity or is it best to change the ballistic coefficient?

Could my magneetospeed be this far off?? Here is the change I have to make to get the calculator to match my actual findings.

308 2745 fps to 2850

6.5 saum 3090 fps to 3200

28 nosler from 3025 fps to 3250

[sierracharlie338]

Truing your drop data is always required but usually done at 600-1000. I know not everyone has easy access to that range. While all the gadgets in the world can be super accurate there is no replacement for real world data. Your 10-25 FPS difference is no biggie. The 100 FPS swing is a little more concerning. I would really try to confirm actual drops around 600-1000 if possible for the 308.

Confirm those actual drop numbers and then you have trued your muzzle velocity to what your rifle is doing in the real world not on “paper”.


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[HTOWN]

Try the Stretlok and Hornady app.

If your hard data doesn't jive with the app, I'd go with what you have. You could always try another chronograph as well

[howabouttheiris]

Did you verify that you correctly entered your scope height over bore? It can really skew numbers in the 1st few 100 yards.

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[ColinR]

Did you verify that you correctly entered your scope height over bore? It can really skew numbers in the 1st few 100 yards.

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Yes 1.55”

[LWD]

Did you use the atmospheric conditions present when you shot? If not that's another reason your data and the calculator won't match.

[ColinR]

Did you use the atmospheric conditions present when you shot? If not that's another reason your data and the calculator won't match.

Yes I did. I’m thinking my chronograph is off. Gonna try setting up my old optical chrony and see what it says.

[Mike D]

I would lean toward the BC of the bullets being off although that usually goes the other way.


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[XR650RRider]

There is a good chance your 100 yard zero is not really 0. If your really shooting 1/2"-3/4" high and you use 0", then it will skew your downrange numbers.

[GigEm'15]

Have you checked your scope? Put it in a vise to be sure that 16 clicks makes 4 moa of movement? Does it make 4 moa consistently?

Some bullet manufacturers aren't truthful with their published BC's.

[ColinR]

I forget what they call it but I have done the test where you dial to shoot each corner of the paper and then return to zero and they all tracked true.

[ColinR]

Thanks for the info. I'm going to shoot over a different chronograph and see what it says as well as shoot out further and see what that does as well.

[txagyotebuster]

I've always had to "true" my velocity to get it to match the actual drop no matter what the chronograph says. It does give a good starting point though

[howabouttheiris]

Yes 1.55”

Hmmm...

IMHO, the difference in the calculator is too much to make up with BC, temp, etc at that range.

It could be velocity, but it would have to be >100 fps off. Verify that the jmb calculator is set to 0 ft and not the default 10ft.

The only thing that can make that difference at 200 is scope height over bore. Make sure you are measuring this center to center accurately, as with a scope height increase of just ~1/2" the numbers line up exactly.


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[sierracharlie338]

I forget what they call it but I have done the test where you dial to shoot each corner of the paper and then return to zero and they all tracked true.

Box test is what I've always heard it called.