Help with a drop chart....

[338 dude]

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

View attachment 225788View attachment 225789

 

[338 dude]

I don't know why it rotated the photos and I do not have any info on the game king bullets

 

[338 dude]

 

[ShtrRdy]

That all seems pretty far off. Do you know someone who would let you borrow a chrono?

 

[Colin78]

That all seems pretty far off. Do you know someone who would let you borrow a chrono?

I have an older optical chrony. Just gotta find it.

 

[Warbird2006]

You do not do anything wrong. You always verify your drop. If you have a place where you can shoot further, 'walk' the rifle through all distances and record everything, that's your data then. That's how the shooters used to do that in the world without computers. Good luck.

 

[DJ Morrison]

To measure your scope height: Measure distance from top of bolt to bottom of scope tube..measure diameter of both the bolt and scope tube. divide in half add together along with the distance from top of bolt to bottom of tube.

 

[swampbug]

To answer your question about do you change velocity or bc to true actual drop, for your circumstances it would be velocity. Although as everyone has suggested, somewhere your input is wrong. I would suggest confirming all your input data AND zeroing at 100 yards as precisely as possible.

 

[25WSM]

Maybe all your inputs are put in correctly but one of them is definitely wrong. Probably speed because it's hard to mess up scope height and bc is a known variable.
Shep

 

[thwatson2]

My bullet drop calculator shows your numbers exactly for 308 And 28 nosler? I don't have Sierra BC number so can't do your 6.5. I use 2 in for scope height in that I find that closer to my real world shooting. But adding 1.5 as others suggest changes it very little, like .2 MOA

 

[BrentM]

Are you sure your scope is tracking true? Are you sure you were parallax free and eye alignment is dead on?

When I mess with the 6.5 Saum data, for my rifle it shows 3.0 moa. I am running a 6.5 PRC and left the rifle profile alone and just added the bullet.

 

[swampbug]

On my 6.5CM, a .2 MOA difference at 300 yards is .6 inches and 56 ft difference in velocity. Just saying, if all your data is correct, but you are off on your true zero and actual impact measurement at 300 yards. You will get a different velocity.

 

[Trailblazer2330]

Following. Interested to see what you go with. When developing a chart for my 300wm, my drop chart called for more than was needed in the first 300yds. But once i got out to 450-500yds, the drop chart was spot on. If posible, maybe try a further range to see if something similar happens.

 

[azarcher10x]

Scope height is the most critical measurement for having your chart read correctly, assuming the chronograph is accurate. That measurement will affect close range (under 100) and midrange, even if a long range, say 800, is accurate. If everything is correct, I'd be sure you're using the correct absolute pressure from a meter, and not what the local weatherman says. I have Zeiss RF binos, and couldn't seem to get the Zeiss app chart to agree with the RF. I physically knew 800 yards was 12 moa. I was updating with the local weather. That was the problem. Once I used the absolute pressure from the built-in weather station in the RF to the app, every thing agreed.