Putting real numbers up to chrony and online ballistic tables.

[Bayourambler]

I am trying to get a dope card made up for a round that I reload for my 308. By using published bc for the bullet and my chrony numbers, the federal ballistic chart is not close at all. Is this common? My sd numbers on this round run around 14. I guess I will do some test shots at different yardage , and maybe come up with a velocity number to plug on to ballistic calc that makes things closer ? I'm guessing all things being equal, the chronograph is way off. It says the bullets run 2715 fps. According to what the come up was at 375 yds, the only velocity number that worked in the calc. was 2900. Is the chrony that far off!?

 

[Fine Figure of a Man]

I've had similar experiences.
Cronys and charts may get you close but all that matters is what your rifle does.
You have to shoot and take notes.

 

[hdwhit]

Bayourambler wrote:
...ballistic chart is not close at all. Is this common?

Yes.

You have to understand what a ballistic coefficient is, which one you're talking about (there's more than one), how it was derived and the circumstances in which it will be used. Basically, the thing to keep in mind is that Ballistic Coefficient published in the manual is either a composite number or a number derived at a particular point in the trajectory that is not going to be the same throughout the bullet's entire trajectory. This thread from back in May 2017 is a pretty good treatment of the subject.

https://www.thehighroad.org/index.p...ing-of-bc-the-wrong-way.821175/#post-10546694

Is the chrony that far off!?

Probably not.

Light sensing chronographs generally don't have a provision for calibration and so the number shown on the screen should be regarded as WRONG in an absolute sense. But, they should generally be regarded as CLOSE ENOUGH. Light sensing chronographs use an electronic timer to determine the time between when the first sensor was tripped and the second sensor was tripped. The electronic timer is sensitive to voltage, so make sure you have a fresh battery. The timer is also sensitive to temperature, so if your range is hotter or colder than it was on the day the chronograph was built and calibrated, the number will be a few percent off.

 

[Bayourambler]

Yep, the math should add up on a chart though. After I get a few confirmed come ups, just keep trying a velocity that matches up.

 

[1stmarine]

Hi,
you might try other chrono to see what you get. Normally they have an error but not so much to matter.
Regarding the ballistics the first thing is to have a reference coefficient for your bullet and then start from there.
Advertised ballistic coefficients are not always reliable and one must correct them. Some manufacturers and
software do a better job at this and provide confirmed values in different forms and formulas and through the different speed spectrum.
Many free or inexpensive calculators do not do a great job at this and calculations lack precision and adjustment.
Get as much data as you can reliably and then with a decent calculator you can get to work on the math/trajectory
given your precise atmospheric values. The wind is also very important as not only affects wind but drop too.
I would say don't take the current values from local websites and measure with a kestrel before you pop them
in. then you can make up one with what you consider to be the average conditions and perhaps a 5mph or 10mph
full value wind. Something that is easy to divide or multiply depending on avg. conditions.
Through testing at different ranges you should be able to verify the ballistic coefficients and adjust accordingly.
What bullet are you using and what is the speed and rate of twist?

 

[Bayourambler]

This load is for my son. Its 125 grain Sierra Pro Hunter. Barrel is 20" 1:12 twist.

 

[.308 Norma]

According to what the come up was at 375 yds,

I'm not familiar with the term "come up." Would you mind explaining?

 

[Bayourambler]

I'm not familiar with the term "come up." Would you mind explaining?

How much MOA up to put into the scope in order to adjust for bullet drop.

 

[444]

What ballistic calculator are you using ?

The one I am using as a cellphone app allows you to correct your come ups. Then it asks you if you want to correct the velocity or the ballistic coefficent. I can measure the velocity with my chronograph, so when there is a difference, I say to correct the BC. And it seems to work. As was mentioned, I don't really care what it corrects as long as it gives me the correct numbers.

FWIW: I downloaded the app and plugged in the numbers that I had; that I mainly got off various webpages or things that I measured. Then when I did load testing, I plugged in a known velocity value that I measured. When I went to the big range, I started at 500 and the dope the app gave me was 2 minutes low. I corrected the app and then went to 1000 yards and the new dope was right on the money. Went to 1200 yards and got a first round hit with the dope provided by the app. So when I made that one correction, everything fell into place in the real world.

 

[.308 Norma]

How much MOA up to put into the scope in order to adjust for bullet drop.

Oh, okay. Thanks!

 

[1stmarine]

This load is for my son. Its 125 grain Sierra Pro Hunter. Barrel is 20" 1:12 twist.

This one right?
https://www.sierrabullets.com/store/product.cfm/sn/2120/308-dia-125-gr-SPT
Sierra do a pretty good job at estimating and measuring ballistics coefficients.

In terms of your conditions what data are you using? ....
Speed: 2715 avg?
Distance to zero:
Temperature:
Altitude:
Humidity:
Barometric pressure:
Wind direction and speed:

When you said you had to come up in MOA how much was that at 375 yards in order to get on the target?
Are you holding the correction for the shot or actually adjusting the scope?
What scope are you using make and model and what magnification setting?

Initially by introducing some variables just with the limited information that we have this is what calculators say....
I used temp: 2715fps, 60F, 555ft ASL, 10mph wind, Pr.29.53hg, etc...

Federals online calculator (kind of barebones)





Hornady's online





Aimtite Calculator. This is a trusty one and it says 8.27MOA at 375 yards that is in line with free/online calculators.


thoughts?

 

[jmr40]

What velocity does the load manual say you should be getting? Is the expected velocity closer to 2700 fps or 2900 fps.

 

[Bayourambler]

This one right?
https://www.sierrabullets.com/store/product.cfm/sn/2120/308-dia-125-gr-SPT
Sierra do a pretty good job at estimating and measuring ballistics coefficients.

In terms of your conditions what data are you using? ....
Speed: 2715 avg?
Distance to zero:
Temperature:
Altitude:
Humidity:
Barometric pressure:
Wind direction and speed:

When you said you had to come up in MOA how much was that at 375 yards in order to get on the target?
Are you holding the correction for the shot or actually adjusting the scope?
What scope are you using make and model and what magnification setting?

Initially by introducing some variables just with the limited information that we have this is what calculators say....
I used temp: 2715fps, 60F, 555ft ASL, 10mph wind, Pr.29.53hg, etc...

Federals online calculator (kind of barebones)

View attachment 762990



Hornady's online

View attachment 762989



Aimtite Calculator. This is a trusty one and it says 8.27MOA at 375 yards that is in line with free/online calculators.
View attachment 762987

thoughts?

That was exactly what I saw on my calc. I was shooting high. Actual come up was 6.75 MOA . 3 bullseyes after that was dialed into the scope.

 

[Klint Beastwood]

You need to true it. I just skipped over most of the the replys so my appologies if someone covered it.
There's a few reasons for it, between natural human error and the long decimal math not adding up to your firing solution. Assuming all is consistent.
In addition, there's more error in chronos the people might believe but I'll leave that topic alone.
I've said it before and I'll stand by it, you are going to want to use a dope book to start. There was nothing wrong with it before software and there is nothing wrong with it now.
Take your real world results and right then down. The true the MV as close as you can to the real world results. If you can only get close then true to the bc. This usually has to be done and ways as the higher or lower the mv the bc increases or decreases. Bc changes throughout its entire flight. If anyone wants and dope book material I have stuff on google drive that you can print off. I have some pretty nifty cheat sheets. I'll have to send you what I have on here though until I can delete some of the school information from some of it as I have pages of compiled stuff from all over, including some us and foreign military schools I attended. Just pm me and let me know if you are interested

 

[1stmarine]

If you provide more data that would be great but this is what I am thinking....
Most likely your error measuring the speed is much bigger than we normally find.
I am saying this because I don't know what load one will have to make in order to produce 2715 with a 125gr bullet.
From the top of my head I cannot think of any tipical powder that will produce that w/o going behind a reasonable minimum.
So the first order of business is to find another chrono and find out the actual speed.
I would be surprised if you are not in the digits your estimated trajectory is suggesting.
I was asking about the scope and if the scope was checked for tracking and it is true and also if you were holding values
since in a 2nd focal plane the moa measure in the reticle will be affecting by the magnification .. but you probably already
know that.
Ask around the range or friends and I am sure you will find another chrono to verify speeds.
But in the end there is nothing like the confirmation on the target at different distances that is the final true meausure.
IMO I would imagine you also want to know what is going on with your chrony if it is that off and perhaps fix the problem
or get a new one.
I hope this helps.

 

[Slamfire]

I level my chronograph with a bubble level and I make sure that I am shooting precisely in a straight line over the sensor. Any off axis error and you can't trust the numbers. I shoot a "calibration" load over my Chrony to verify alignment. It is a load that I have shot before. I have shot bricks of Remington target 22LR, bought a case of the stuff. I also shoot factory center fire ammunition for a reality check against my reloads.

I trust the velocity I get over my chronograph over any tables that give MOA increases with distance. I also trust my chronograph readings against any book or box values published for that ammunition. I don't trust their calculations because I don't trust their numbers. I am of the opinion that factory numbers are inflated, because shooters want high numbers. Since 99.99% of the buyers don't actually chronograph or shoot the ammunition at range, no one complains. I don't use their data to predict elevation come ups. I go to the local range and shoot on a target, get my 100 yard zero. Then I drive to CMP Talladega and shoot my sporter rifles at 200, 300 and 600 yards. I trust the MOA reading on target much more than any book value of what it is supposed to be.

 

[Klint Beastwood]

No matter what you get on the chrono it's not truely going to align with your data. Even if you get it close, say hitting steal all the way to 500y, your drop won't stay lined up without fudging the numbers from around 800y and beyond. Having software that allows you to true will make it easier but you can plug and play as well. Making having your drop at known distance more benefitial so you can do it from the convience of your house.

 

[Klint Beastwood]

What ballistic calculator are you using ?

The one I am using as a cellphone app allows you to correct your come ups. Then it asks you if you want to correct the velocity or the ballistic coefficent. I can measure the velocity with my chronograph, so when there is a difference, I say to correct the BC. And it seems to work. As was mentioned, I don't really care what it corrects as long as it gives me the correct numbers.

FWIW: I downloaded the app and plugged in the numbers that I had; that I mainly got off various webpages or things that I measured. Then when I did load testing, I plugged in a known velocity value that I measured. When I went to the big range, I started at 500 and the dope the app gave me was 2 minutes low. I corrected the app and then went to 1000 yards and the new dope was right on the money. Went to 1200 yards and got a first round hit with the dope provided by the app. So when I made that one correction, everything fell into place in the real world.

That is the way to true. +1
Which app do you use AB?

 

[jmorris]

FWIW the actual BC of a projectile changes depending on the speed it's going. Also lots of environmental factors not accounted for and maybe some human error too. Ever seen the same bullet, gun, etc in a reloading manual then they come up with two different powder charges, of the same powder and get different results?

Like 27.7gn of 296 giving 1500 fps and 27.8 of H110 only giving 1300 fps and knowing they are the exact same powder with different labels stuck on the container...



Charts get you close, then you have to confirm. Same reason cook books give you a time and temperature then tell you to confirm with a meat thermometer before your done.

 

[444]

A cool thing about the cellphone app is that you can also set it to adjust the dope for current temperature in real time based on the weather data it gets from the cellphone. I believe it does this with elevation also. I am not sure if it actually corrects this automatically on it's own or it depends on you shooting in various weather conditions and then it establishes a model of changes due to temperature or elevation or how exactly it works. I am only in the learning stages of this whole thing.
I know you can also enter in both a G1 and a G7 BC.

Now I realize that all this stuff is only as accurate as the data you are providing it: garbage in, garbage out. But, I think at some point you have to just say, this is good enough data. In other words, yeah the current temperature from the phone might not be exactly correct but how correct does it need to be ? And my answer is that when you are hitting the target using the dope provided by the app, that is correct enough. I guess I should also point out that I am not shooting on paper, I am just ringing steel and in reality, I might be off by a inch or two (at the furthest ranges) but for my purposes, this is good enough.

I am using a Magnetospeed chronograph, so I frequently just leave it on the gun the whole time I am shooting. When I get a average velocity, it is an average of 40 or 60 (or whatever) rounds fired and not just a single shot fired over a chronograph or one five round group or whatever.

I am a realitively new to semi-serious longish range shooting but I have seen for myself, a lot of the stuff I am reading about in these threads. As was mentioned, it is important to keep a good old fashioned dope book. At these various ranges I go to, where again, I shoot steel, I have a dope book where I write the actual come ups I used to get on target at the various ranges. The cellphone app is sort of just to get me close. One very good reason for ME to keep a good paper dope book is that I am just shooting steel at "known" distances. But I don't really know how precisely those distances were measured. In other words, they might say that plate is at 1200 yards, but in reality it might be at 1225 or 1175 or whatever. So, I dial in the dope for 1200 yards and make a marginal hit or miss and think the phone app is giving me bum information when in reality the target isn't really at the range I thought it was. But, if you record the dope you actually used to hit the target, then the next time you are at that range, you have good proven dope. The more data you compile in the dope book, the better. And most of this is almost just guessing if you are shooting at steel without a (good) spotter.

 

[jmorris]

But, I think at some point you have to just say, this is good enough data.

I understand and agree. I was just pointing out why differences can exist. Anything is better than a guess and sometimes they are very, very close. Then someone comes along as says "Close only counts with horseshoes and hand grenadines."

 

[wally]

I don't have a place to shoot past 400 yards so this may not be helpful to you, but what I did was make a good solid 50 yard zero and then shoot a 10+ shot group at 300 yard (longest square range I have access to). I then set up the ballistics calculator with what should be the right parameters for the bullet weight & BC, temp, scope height, etc. and then adjusted the velocity to get the best match with my observed drop at 300 compared to the 50 yard zero. Seems to be working pretty well for counting clicks to adjust for different distances or using mil-dot reticle BDC -- meaning my data for closer and further steel plate targets seemed pretty good. I've been two lazy to dust off the chronograph (PACT) and this seemed like a good way to get started. I'd certainly revisit the issue when I change ammo or find a place to shoot with longer distances.