It started nagging me and can´t physically test it at the moment - if nothing but the powder charge is changed, meaning a faster/slower Vo, how does that impact the zero compared to the original load? Would the same bullet flying faster hit higher or lower at 100y?
Range Report POI zero shift with different load
- Thread starter Lukas Koukal
- Start date
In a simple world yes, a higher velocity with all things being equal would result in impacts higher on paper. Ladder tests show this trend.
However, barrel harmonics come into play and sometimes push groups to where you would not expect.
However, barrel harmonics come into play and sometimes push groups to where you would not expect.
That´s my reasoning to ask for experience; ladder test is properly performed at longer distances, and faster loads hit higher simply because of flatter trajectory. I believe this doesn´t apply at 100m, at such short distance the loads (unless extremely slow) should be equally flat shooting. Bullets will, however, exit the muzzle differently - but how exactly?
In pistol shooting, it´s slower loads that hit higher actually. The recoil is happening before the bullet leaves the barrel, slower bullet leaves it later - that is at a point when the muzzle travelled upwards for a longer time. The engle under which the slower bullet leaves the muzzle is therefore minutely higher.
But how´s this with long guns?
Say the example I have at my hands - .308win loaded with 155grs palma bullets. One load is moderate 2850fps, another rather hot 2950fps. What should be their difference at 100, if any can be witnessed at all?
In pistol shooting, it´s slower loads that hit higher actually. The recoil is happening before the bullet leaves the barrel, slower bullet leaves it later - that is at a point when the muzzle travelled upwards for a longer time. The engle under which the slower bullet leaves the muzzle is therefore minutely higher.
But how´s this with long guns?
Say the example I have at my hands - .308win loaded with 155grs palma bullets. One load is moderate 2850fps, another rather hot 2950fps. What should be their difference at 100, if any can be witnessed at all?
Your 100m elevation zero is comprised of two parts: First is compensation for the bullet drop from the muzzle to 100 yards. With a .308 that drop is about 2.7 inches, or 2.6 MOA or .75 Mils. Second is the mechanical offset of the line of sight over the line of the bore. Assume a sight height of 1.75 inches, which at 100 yards is about 1.7 MOA or .5 Mils. Add those two up, and you get about 4.3 MOA or about 1.25 Mils.
With a faster load, the first part - drop in inches from muzzle to paper - will be less.
That means we must answer two questions:
1) How does it change the zero?
2) How does it change the POI?
With a faster load, the first part - drop in inches from muzzle to paper - will be less.
That means we must answer two questions:
1) How does it change the zero?
2) How does it change the POI?
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Now it sounds more complicated than I first thought for myself; honestly, I´ll shoot the two loads Saturday and I´ll see what they do and how they correlate to each other, but now I´m simply curious and want to better understand this part of ballistics; could prove useful in the future, who knows. I ran the two exact loads through JBM and enclose here the results. It is apparent the 2950 trajectory is flatter to 100m, but only by 1mm at the highest point. Faster bullet also crosses the line of sight for the first time cca 10m later. The calculation would almost say the difference is there, but probably will not be actually measurable in this case?
You have to take into account more than just velocity. If you loom at OCW testing at 100 and 200 yds you will see that your POI will change as your charge changes. Your POI may stay the same over 1-1.5gr but then it will move once you go higher. Say you loaded from 41-45gr varget under 178gr amax. You might be dead on from 41-42.5 and then 43-44 may shift to the left and up 1/4". 44.5-45 might be centered but to the right a bit. Wish I was home so I could load a pic of what I am talking about.
At what distances is a .308 trajectory flat for purposes of POA/POI?
For example, is your 96yard zero different from your 104yard zero?
Uncheck the box "Elevation Correction for Zero Range " in JBM. This will give you the level barrel bullrt flight path and it will show you the different POI based on MV.
[MENTION=66602]waste_knot[/MENTION]: did just that, thanks for the suggestion. Now JBM simmulates a bullet fired horizontally; slower bullet drops 6,9cm to 100m, faster bullet drops 6,4cm to 100m. That equals 0.05mils difference between the two. I now believe that if any larger difference is observed between the loads, then it´s caused more by barrel harmonics and how the bullet exits the muzzle than by the external ballistics of the 100m flight.
Add to the above that differences in recoil will affect where the barrel points when the bullet emerges. This brings the human body into the equation. It could be that more recoil results in a higher line of departure, and it may not. Therefore, a higher charge does not demand either a higher or a lower impact. You can't figure this out on paper unless you have all the facts, and not all of those facts will be consistent once the human body enters the arena.
Greg, that´s what I understand from pistol shooting as I´ve mentioned before. That is, letting the rifle to recoil freely as compared to a hard hold could introduce a greater margin between the two POI, correct?
Not necessarily.
Meaning one variable (upwards recoiling rifle) could negate another variable (bullet leaving the barrel sooner), leaving no evidence in the target, yet knowing the factors still exist..?
I don´t want to have precise data without shooting it, indeed the ultimate truth lies with where the bullet hits. But dang, do we have to agree that there´s no way to at least make a pretty accurate theoretic estimate?
I don´t want to have precise data without shooting it, indeed the ultimate truth lies with where the bullet hits. But dang, do we have to agree that there´s no way to at least make a pretty accurate theoretic estimate?
Some great information here. Thanks to all of you.
Verified reality = the faster load hitting 1.4MIL higher. No difference in windage.
Is this at 100yds, like your example??? That seems like a huge factor. Is this with fully controlled recoil?
Yes, I am still talking the above-referred cartridges. Loaded 3 of each back-to-back in one mag (1 slowe, 1 faster, 1 slower...) and shot them in one string on one POA. Measured 14cm difference between the groups at 100m. Seems barrel harmonics play even bigger role than I wanted to admit; in that case, the difference will also be different in other rifles, i.e. it isn´t possible to predict at all.
I stand corrected on the first assumption I´ve made when I started this thread; the results can´t be anticipated with acceptable accuracy.
I first thought if I have the rifle zeroed with one cartridge and then I am forced to shoot another load without being able to test-fire it first, that I could +- estimate the change in POI and dial in the exact difference in a shot or two like during a competition timed sight-in, for example. It can be done to redial from one load to another, but physical testing of the loads is the only way to go and to actually know.
I stand corrected on the first assumption I´ve made when I started this thread; the results can´t be anticipated with acceptable accuracy.
I first thought if I have the rifle zeroed with one cartridge and then I am forced to shoot another load without being able to test-fire it first, that I could +- estimate the change in POI and dial in the exact difference in a shot or two like during a competition timed sight-in, for example. It can be done to redial from one load to another, but physical testing of the loads is the only way to go and to actually know.
Interesting data. Please keep us up to date if you develop more data that shows trends here. I bet there is a way to figure it out, but you probably have to collect some data on that rifle and then figure out the correlation. After that, you can probably figure the changes most of the time, and at least get close in a new development. It might, however take way too long to get a handle on it to actually make it usable---your barrel/throat/other changes might preclude being able to develop actionable data conclusively.
No way that's the load. It's you, behind the rifle.
It's fairly easy to predict the results, verify them, and keep track of two zeros.
5" difference at 100m? I don't think that's the load. Normally my test loads will span about 5gr of powder and never a change in POI of more than 1" @ 100yds. I really don't see how you could get 5".
It´ll need repeating for sure; this is what I have now. Graham, this was not two groups shot separately, no change of position etc.; all I did was shoot at the same point and work the bolt. I shot them in one string loaded as pair/impair numbers in the mag. Result = two .5" groups 14cm apart. Dunno. The faster load is 108% MAX powder according to QL, maybe the higher pressure = greater barrel whip in the node... melloyello is right that I´d have to work with more loads to actually track any trend.
Think about it this way - if the barrel vibration is kicking the bullet 5", your group is going to huge because even a slight variation in velocity will have those things spraying all over the place. Add one more vote for "something else changed".
If your groups were that tight, but 5 1/2" apart at that distance, then there won't be any trend to track.
I don't wish to appear jaded, but most of this discussion probably takes longer to read through than it would to fire a few rounds of each as a comparison.
I recognize the value of mathematics and computations, but in the end, only the rifle will speak the truth; most else is purely conjecture.
Several decades of handloading and serious shooting have taught me to cut directly to the chase, and to save the math for when all else fails.
Shoot the gun first, then deal with what questions remain; conjecture never defeated a target.
Or as my eye-deel, Brett Maverick, once put it; faint heart never filled a flush.
Greg
I recognize the value of mathematics and computations, but in the end, only the rifle will speak the truth; most else is purely conjecture.
Several decades of handloading and serious shooting have taught me to cut directly to the chase, and to save the math for when all else fails.
Shoot the gun first, then deal with what questions remain; conjecture never defeated a target.
Or as my eye-deel, Brett Maverick, once put it; faint heart never filled a flush.
Greg
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Indeed it does, Greg... if there is an opportunity. My nearest range is 1,5hrs drive, I can get there only on some weekends and that´s only 100m. I´m regularly travelling 500+ km to shoot long range. And that is the real deal behind the question - if there were a way to do the maths instead of repeatedly verifying zero (which i often simply cannot manage before an upcoming comp), it would be of great help for me. Practical options are not always favorable, you see...
Best fortune finding your answer and Happy Easter to you and all here...
Greg
Greg
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Any of you guys have some links to good articles on barrel whip and harmonics affecting POI?
TAG hope to become a loader someday 
Not a link, but Harold Vaughn's book, "Rifle Accuracy Facts" is about as in-depth a discussion of it as I have ever seen.
ladder test on a heavy Lothar Walther new-rebarrelled, free-floated, bedded .222 Anschutz, 36x scope, 200mt.,from the bench: the hotter the load, the lower the impact point_ (no lateral stringing)
can be am I...
can be am I...
As I understand how harmonics affects POI, when combustion occurs, a shock wave travels up and down the barrel, from chamber to muzzle, several times before the bullet exits the barrel. When the shock wave is at the chamber end of the barrel, the barrel muzzle is at its calmest state. Conversely, when the shock wave is at the muzzle, the muzzle is oscillating. Incremental changes in powder charge weight produce incremental increases in pressure, which produce incremental increases in velocity, which produce incremental changes in the speed at which the bullet travels down the barrel, which produce incremental changes in the amount of time the bullet spends in the barrel.
Since the shock wave is traveling up and down the barrel faster than the bullet is traveling down the barrel, there is a narrow window where the bullet exits the muzzle while the shock wave is at the chamber end of the barrel. This is the "accuracy node." Conversely, the narrow window where the bullet exits the muzzle while the shock wave is at the muzzle end of the barrel is the "scatter node."
The "trick" in doing OCW and ladder tests is to find the charge weight that results in the bullet exiting the barrel while the shock wave is at the chamber end of the barrel. This will produce your accuracy node and your best results.
Thanks Bob that is some good information similar to the Optimal Barrel Time Paper.
Yes there is POA vs POI shift with different loads and thus velocity even at 100 yds because of barrel harmonics.
I'm not smart enough to examine and evaluate the technical scientific details of such discussions(although this does intrigue me), so I just shoot my possible loads and keep pictures in my cell phone of how the different loads I plan on shooting group in accordance of one another, then if I'm changing rounds I adjust the turrets accordingly. I have loads that shoot right of center 1.5", a load that is dead zero, and a load that shoots high 1" left 1/4". My variances in load POI's @ 100yd
s is interesting to me because my 22-250 has very little variances at such short distances, and my friends .308 shorty has almost no difference of POI for 5 different factory loads. Anyhow, ill step back and continue getting my learn on....
s is interesting to me because my 22-250 has very little variances at such short distances, and my friends .308 shorty has almost no difference of POI for 5 different factory loads. Anyhow, ill step back and continue getting my learn on....
