Drop charts?

[JohnD]

I am trying to build some drop charts for an upcoming varmint hunt and I'm having a hard time getting my numbers to agree. I am using the Berger ballistic chart and my 100 yard site-in does not agree with the 300 yard POI. Or vise versa. The only way I can make sense of this is manipulate the velocity on the chart so the numbers will agree. Velocity being the varible that seems to make the most difference. I have tried different elevation values but they do not change the POI as much as what am seeing. The only thing I can come up with is these charts use muzzle velocity and the chrono is measuring this velocity several feet in front of the muzzle. Regretfully, I did not measure the exact distance from muzzle to the first screen. I'm I missing something else here? Should I build the charts using the manipulated velocities and ignore what my chrono said?

Thoughts, opinions and advice is most welcome.

John

 

[XBBR Shooter]

I am trying to build some drop charts for an upcoming varmint hunt and I'm having a hard time getting my numbers to agree. I am using the Berger ballistic chart and my 100 yard site-in does not agree with the 300 yard POI. Or vise versa. The only way I can make sense of this is manipulate the velocity on the chart so the numbers will agree. Velocity being the variable that seems to make the most difference. I have tried different elevation values but they do not change the POI as much as what am seeing. The only thing I can come up with is these charts use muzzle velocity and the chrono is measuring this velocity several feet in front of the muzzle. Regretfully, I did not measure the exact distance from muzzle to the first screen. I'm I missing something else here? Should I build the charts using the manipulated velocities and ignore what my chrono said?

Thoughts, opinions and advice is most welcome.

John

You didn't say what bullets you were using and you did not say how much disagreement there was between the program and actual POI.
Keep in mind that the bullet companies do NOT use the same methods to assign BC numbers to their bullets.

For example, the Sierra 55 BlitzKing has a BC of 0.272, and the Hornady 53 V-Max has a BC of 0.291...
... but the BlitzKing shoots flatter (by a tiny bit) than the 53 V-Max, when launched at the same velocity.

Also, make sure your scope height above the bore line is accurate.

If your POI doesn't match the predictions, change the BC, not the velocity.

 

[germanlongbow]

I shoot at known distance. Start at 100 yards. Then move out in 100 yard increments. Measure muzzle velocity. BC. I like to know what my exact bullet drop to turret movement. I plug in the numbers to my program. Correct for both MV and BC. Has worked well for me out to a mile.

 

[JohnD]

Thanks for the replies. The bullets I have been using are the 105 Hybrid and a 95 gr. SMK. I did measure my scope height best I could and made an adjustment there. It did make some difference but not enough. The actual difference on the 95 gr. is the chart is showing about 1 3/4" more drop at 300 yard. The Berger a little less. Is this within an acceptable range of due to all the variables?

Thanks again, John.

 

[Rem06]

Thanks for the replies. The bullets I have been using are the 105 Hybrid and a 95 gr. SMK. I did measure my scope height best I could and made an adjustment there. It did make some difference but not enough. The actual difference on the 95 gr. is the chart is showing about 1 3/4" more drop at 300 yard. The Berger a little less. Is this within an acceptable range of due to all the variables?

Thanks again, John.

take it to 400 and 500etc....your bullet hasn't even really started to "drop" yet.

 

[MarkS]

John,

Thanks for the replies. The bullets I have been using are the 105 Hybrid and a 95 gr. SMK. I did measure my scope height best I could and made an adjustment there. It did make some difference but not enough. The actual difference on the 95 gr. is the chart is showing about 1 3/4" more drop at 300 yard. The Berger a little less. Is this within an acceptable range of due to all the variables?

Thanks again, John.

John,
You are doing everything right. I go by POI info. I just shot my new 6BR barrel Friday to make my chart. I did not Chrono. I zeroed at 100 yds, then shot 200, 300, 400, and 500 yds. I knew, I would be going between 2800 to 2900 f/s. I adjusted speed until I matched the POI on JBM. The speed came out at 2890 f/s.

Mark Schronce

 

[Lapua40X]

John, I usually deduct 10% from the bullet manufacturer's advertised BC and begin with that, combined with my MV. I shoot in 100 yard increments using that data, out to a least 600 yards, and then adjust my dope sheet to match true performance results. I have never found a "program" or ballistics calculator that could predict accuracy performance with any assurance. My suggestion is that you always base your dope on actual performance on target and forget about the promises offered by the calculators.

 

[wapiti25]

The air density the day you shoot will change, as well as mirage, wind (head wind and tail wind). Any wind seems to change my drop ( increased drop) at 600 yds.

 

[IdahoSharpshooter]

did you set the temperature for where you are, or where you will be shooting?

 

[CharlieNC]

I'm not familiar with that program but regarding altitude most have a provision to use the associated default barometric pressure and temperature, or enter the actual values. Easy to overlook, with major consequence.

 

[jcampbellsmith]

Something to consider is the terrain you are shooting over. Is it flat out to 300 yds and beyond, or are there any features between you and the target. I shot at 400m on a formal landscaped range last night and was comfortable with my drops, however some of the field locations I shoot in aren't flat and I no longer try to draw conclusions on my drops when practising there.

Regards

JCS

 

[Sniper338]

Chrono your load and record ambient tempatures, make sure the ammo and barrel are the same as ambient tempatures... i record data in every tempature range throughout the year...

plug it into a ballistics calculator, this gives you a starting point. Measure out distances, shooting flat.... record your actual dope at given distances you want. Ballistics calculator just gets you in the ball park.

 

[JohnD]

Thanks for the recent replies. Didn't realize this post was still going. Got back from my PD trip and did not shoot many. Did set some water bottles up on an old dirt pile @ 600 yards. My come up clicks from a 100 yard zero are as follows:
107 SMK @ 3050-24 clicks
105 Hybrid @ 3125-22 clicks
95 SMK @ 3250-20 clicks
These numbers don't come close to agreeing with the drop charts I made. I get that barometric pressure, temp, terrain, etc can make a difference but I used another program that is unavailable now and was amazed at how close it was even while guessing at some of the parameters.
I'm now thinking my scope is not calibrated properly but at one time it was perfect as I did the "box" test at 100 yards and was spot on.

Thanks again, John.

 

[DocUSMCRetired]

What kind of chrono is it? Have you checked out our Chronograph Performance Review Article?

If you are using a known tested and accurate BC like Berger, then MV needs to be adjusted.

Errors in atmospherics are not to be taken for granted. 15 degrees is equal to 3% error in DA. DA and BC have a direct correlation. Meaning if your DA has 3% error, then it will act like a 3% error in BC. The factors that make a 3% error are 15 degrees F, 687 ft altitude, or .87 inHG. These are compounding, not combined. So all 3 of these will equal 9% error in BC if guessed wrong.

Using drop to calculate MV is also not ideal. It can be done, but needs to be done carefully. What is your SD? An SD of 10 is equal to about 1 MOA of spread at 1000 yards. So any higher, and your compounding more problems. When you shoot for MV you need to shoot at 800 yards minimum and that's with a 308. It varies for different calibers, but you need to be trying to tune for MV at Mach 1.2.

It probably is a good idea to check the turrets. Its always important to know exacly how your turrets are tracking, but it is easily calibrated out with MV if their is any error.

We have a lot of guiders here: www.abmediaresources.com. However I highly recommend the ballistic calibration article, tall target test, and then moving through the rest. Once that is done, we have a free ballistic calculator you can create your cards on at www.appliedballisticsllc.com/ballistics

 

[CharlieNC]

Looking at your drops for various bullets/loads, are you assuming these have the same 100yd zero?