Spin drift

[BHarvey]

I do the majority of load work at 400 with a little at 600.

My 400 yd groups are anywhere from 1-1.5" with 1.25" being average. 600 yd groups are under 3" with some as low as 2".

At 1k the groups all are under moa and is very consistant.

My 100 yd groups are in the .3"-.6" range depending on how I'm shooting that day.

I'll shoot a few this weekend at 400 and post them along with my 100 yd groups.

 

[rstreich]

...My view is that all ballistics are governed by the laws of physics and that you can't break the laws, or bend them, even if you tried. Whenever the actual results on paper seem to contradict the laws there has to be a reason and I believe that reason is either environmental or the shooter himself....

The problem is that in order to apply the laws of any system you first have to be able to accurately specify all of the variables in the system. From what I've read and learned on the topic, this system,an explosive charge propelling a bullet down a barrel to a distant target), even without the introduction of the shooter and the environment, is currently too complex for us to be able to accomplish exactly that. So, when something seems to break the laws, another explanation is that our model of the system, doesn't include all of the applicable laws,the variables) or we have the values wrong.

Take your example of high ES loads still shooting good groups. One possible explanation is that the barrel harmonics compensated for the velocity differences in the load, causing the bullets to exit the barrel at slightly different trajectories. I have no clue whether that's a truly viable possibility, but it's a good lead in to the point that we can't even specify all of the variables for one component in the system--the barrel. We can derive models with random distributions of varying grain structure, density, and all of the other things that we know affect wave propagation. We can even throw in some of the things that we believe affect wave propagation. But we can't reliably predict the results for a given barrel.

I'm also fairly convinced that we know less about the dynamics of a bullet in flight than we do about wave propagation in solids. I'm not a ballistician. My opinion is based solely on the fact that we cannot seem to take any given projectile and accurately predict its trajectory for a given velocity. Instead, we have to rely on experimental results. Correct me if I'm wrong here as this is a derived opinion based on possibly incomplete or incorrect information. But, if we truly cannot accurately predict the trajectory of any given projectile, then there is still quite a bit we don't yet understand about the physics of a bullet in flight.

I agree that the laws of physics are inviolable,at least, this side of a black hole), I'm just unconvinced that we know what all of them are, and completely convinced that we don't yet know how to apply them accurately in complex systems.

Sorry if I'm digressing into an uninteresting subtopic of your post, Ray, but I guess I just felt like beating that drum all of a sudden. I'll stop now before the neighbors call the police.

robert

 

[Raptor]

BHarvey,
Are you shooting 5 shot groups??? What caliber are you shooting?? Thanks!!

 

[BHarvey]

Yes, 5 shot groups. 260 with 139's at 2760 fps. Cases are Lapua 243 and are necked up then turned to give .002" clearance. The RP cases shoot reasonably well but they won't shoot with the Lapua cases.

 

[Cheechako]

Robert

That was a great post. You need to chime in more often.

I totally agree with what you said. Back on page 1 of this thread, which started out about spin drift, I said that ballisticans themselves can't quantify spin drift because of the unknowns of bullet dynamics. With a lot of testing they could probably make a stab at it for a particular bullet but it would be cost prohibitive for their entire lineup. Even our own Federal Government, with seemingly unlimited resources, hasn't shed much light on the subject.

And I think that barrel harmonics and accuracy is just starting to get the attention it deserves. The short range benchrest shooters are having big-time debates about tuners,which the rimfire guys have been using for years) but new ideas and theories are are always viewed with skpeticism by shooters. I asked Bharvey about whether he chronographed while shooting because I've always felt that there is something at play that may be far more important than ES and SD at long range. What he reported seems to bear that out but we're a long way from knowing exactly what it is.

Testing theories at long range is one of the biggest obstacles we are faced with. Shooter and environmental factors tend to be so overwhelming that it's impossible to seperate the gold from the sand. And maybe that's a good thing. I've always felt that when we start shooting one caliber groups our interest will stop. I already see signs of that in a lessening interest in short range,100 yards) and a growing interest in long range Benchrest,200 yards +). Stripping Mother Nature bare and learning everything about her is like watching a stripper,not that I do that). Once her/his clothes are off our interest drops. Well sorta.

Ray

 

[sierra22]

Agreed, a lot of good replies. That's the beauty of a discussion with good people; it'll take interesting turns. I'm very happy with this one

As to quantifying spin drift:

100m - 0,2cm
200m - 0,7cm
300m - 1,8cm
400m - 3,4cm
500m - 6,0cm
600m - 9,7cm
700m - 14,8cm
800m - 21,8cm

This is for 7,62NATO,308) with 9,45grams M63 projectile at 870 m/sec through 1:305mm twist and in standard atmosphere. Measured data, by radar I believe. To my understanding, it can't be calculated as one needs some variables that can only be measured.
,Data has been published in open publications, so this shouldn't be restricted).

Numbers spin from barrel. If you're zeroed at e.g. 600m spin will be 0 there and about 12cm at 800m,21,8 - 9,7).

Spin will as I understand it be different with different cartridges.
---
As someone wrote, I agree very much that there probably isn't much that can be hidden from science regarding bullet flight and alterations due to physics. Diverting the resources to investigate however...

Some myths seems very hard to kill, and the BC vs diameter/weight thing Ray mentioned seems absolutely unbelievable for many.

And for the shooter, applying correct law to correct phenomena, not to mention putting it into a system ...well... let's say threads as these would be non-existant if it was easy.
---
As for load development, haven't really done much of it in the way competition shooters due. When limited to issued ammunition, all I can do is call one of the larger training centers, try to get hold of an enthusiastic training officer and ask if they have identified any favorable production lots. Even if I get a number, that doesn't mean I get the ammunition. I must informally ask the local supply officer if he will let me have a look at the lots in stock. If it's there, I'll get it, if not I'll have to make do with whatever. At present there's no system for getting the right lots in the right rifles.:,

As a result of this "load development" has over the years become testing at 100m and 300m,the ranges I can always get access to) and shooting at silhuettes beyond that - any deviation is noticed when measured against expected hit percentage and not groups.,Don't use groups for anything other than a planning tool really, and then the basic "group-increase-proportional-to-distance" to see if group will fit inside the target or not. Hence my earlier and unsuccessfull paper/group digression - "Will I likely hit the target?" is the relevant question for me. Then "why not?" and "how can I eliminate the "not"?".)

Private system has become the same, and load development simply stops at "good enough" which is below 20mm@100m,app 2/3MOA?) and hitting a target about 8-15" at longer ranges; range depends on conditions. No good for competition, I know.

Data for wind/temperature/pressure deviations has been meticulously collected and incorporated. I mechanically correct for these, as with spin drift, regardless of distance/group size/noticable effect. The theory is removing all variables possible.
Draw up a baseline for wind, make an "experience-call" for deviations from this baseline,observe, and commit to a target. Any last second deviations are compensated for by hold. Finished at first hit.
---

Anyone notice impact climb/right wind, impact descend/left wind and if so how much?

And more on light please...

 

[rstreich]

Robert

That was a great post. You need to chime in more often.

Thanks, Ray. I appreciate that. I'm still in learning mode here so I don't often feel I have anything to contribute. I've read and enjoyed many of your posts over the last couple of years, so I knew you were one of the people with an open mind.

...The short range benchrest shooters are having big-time debates about tuners,which the rimfire guys have been using for years) but new ideas and theories are are always viewed with skpeticism by shooters....

I shot High Power for a few years long ago. It amazed me how resistant many shooters were to trying something new. It seemed to border on superstition. I haven't been to a benchrest match, so my sample population is a bit skewed, but from reading the forums, there seems to be one key difference with benchrest shooters: if they see someone else winning with some extra gizmo attached somewhere to their rifle, they're pretty likely to try one.

Testing theories at long range is one of the biggest obstacles we are faced with. Shooter and environmental factors tend to be so overwhelming that it's impossible to seperate the gold from the sand. And maybe that's a good thing.

I doubt that we will make much more than incremental progress in this area in the future. As you point out, experimentation is cost-prohibitive and the only likely funding source for experiments, the military, is pretty deeply entrenched in fire-suppression mode. They're more interested in three-shot bursts than they are in accuracy,personally, I always thought Carlos Hathcock's form of fire suppression was more desirable). Who knows, this may change slightly with the renewed interest in marksmanship.

I've always felt that when we start shooting one caliber groups our interest will stop. I already see signs of that in a lessening interest in short range,100 yards) and a growing interest in long range Benchrest,200 yards +). Stripping Mother Nature bare and learning everything about her is like watching a stripper,not that I do that). Once her/his clothes are off our interest drops.

I agree with the one-caliber groups, but with the other, well, sometimes, when all the clothes come off, :love:

robert

 

[rstreich]

And more on light please...

Light is primarily an issue when you are shooting a post front sight. Bright light shrinks the target, diffuse or dim light expands it, so you have to adjust your vertical to account for it. I don't think match sight,circle in circle types) or scope target shooters have to adjust for it other than to change an iris or two.

It's just a guess, but I would bet that the changes that BHarvey experienced with the changing position of the sun were just the result of mirage. I forget where I read about it, but I believe it's called static mirage. The flutters that we see down range and refer to as mirage are just compression waves caused by wind pressure. They create localized areas of refraction that we can discern because we see the image wiggle around in them. Static mirage is the relatively constant displacement of the entire image. We can't see it because it affects the entire image and changes slowly as the light source moves. It's the same sort of thing that allows us to see the sun when it's actually below the horizon.

robert

 

[sierra22]

Interesting...

As I said, don't see much of mirage over here. When it occurs, it's usually in early spring,bright sun on snow) or mid-summer,warm sun on moors). The only thing I know how to use mirage for is direction and change - seeing too little of it to say anything about it with regards to speed.

The only light effects I have,imagined to) noticed is when bright light diffuses target silhuette, I then seem to put the sight on the more "solid" part of the target.

As to climb/descend w wind, I've noticed this only once,again, if I'm putting right label on the phenomena). This was in a very strong right wind,had something in the area of 20 clicks windage) and then I ended up with a turret setting somewhere like 30m short of where the target was,500m target, 470-480 on turret setting, no significant temperature/pressure deviation).

 

[Raptor]

As Sierra22 said this is and intresting topic. I have shot for more years than I care to remember and have been into long range shooting for about 15yrs now.

When I first started shooting past 500yds on out to 1000yds I spent a lot of time getting my dials and wind dialed in for my loads.,Didn't have a good compurter then) The one thing that drove me crazy was that one day I could go out and be dead on with my dials to 1000yds and the next day under nealry the same conditions,Temp and sun) I would be off by a full MOA either up or down. I used to correct for it thinking I had bumped something or somehow the dial was off.

Then to my dismay I would find the next time I would be off the other way as much as I had corrected the day before!!! Then when I started drag racing again I found a neat tool they were using called a weather station. What it did was analyze the air and would give you a corrected density altitude.

In other words you may be at an actual altitude of say 1500ft but becasue of the atmospheric conditions your motor thinks its running in air at say 4000ft. So depending on conditions this would very from day to day and sometimes would change from 900ft DA to 5000ft DA in one day!!!!

Now if you motor thinks its running in 5000ft air and won't make the same power as when its in 500ft air, why wouldn't this also affect how your bullet flys at long range??? In other words the bullet is flying in air that is the same as 5000ft but your actually at 1500ft. So if you have your 1500ft data on you will hit high on the day the DA is at 5000ft and low when the air is ar 500ft.

 

[rstreich]

I've seen the benchrest shooters on BR Central passing around a little windage diagram that shows corrections for different winds. Even with their slow-twist barrels, there is a change in vertical in the correction. There's one attached to a post in this thread. There aren't any values for the correction as I'm sure this varies with barrel, bullet, and load.

robert

 

[BHarvey]

The basic rule of 'lights up sights up' and 'lights down sights down' applies to scopes as well.

At the last match in August, my first shot at 1k was under overcast skies with a center 10x hit. By the time I was ready to fire my second shot, hard sunlight came from behind the clouds to light the face of the target. My first thought was to hold a correction for elevation and then I thought, I have been burned by this before and so I held the same poa and low and behold, my bullet poi was .5moa out from my first shot. Now for shots #3-5, I held the correction I KNEW I should have held for shot #2 and all three went center 10x. Recall, shot#1 was with overcast skies. Shots #2-5 were with bright sunlight and the mirage was the same for all shots.

Brings to mind the saying of "Think long, think wrong"!

 

[sierra22]

Raptor, totally agree. I have a weather station reading QFE,station pressure, not pressure corrected for altitude above sealevel) and a datacard that gives + or - click corrections for comeups. The baseline is whatever conditions prevailed when I set up the card.

For example: 1000 hPa is standard metro pressure as published in ballistic tables and default in same programs. I shoot 1-800m and log comeups with pressure 970hPa. I punch everything into the computer and it comes up with a table that usually pretty much fits my tested comeups. I then alter pressure to 980, 990, 1000,1010 etc and print new datatables. The deviations from the tested table is noted and logged as +/- corrections.

,You are supposed to correct QFE for temperature, but even with great variations the deviation from uncorrected QFE is so small I don't bother.)

Same is done with temperature, only I'm careful to crono ammo at through the year to get Vo changes. I'm then careful to keep ammo temp same as air temp.

This system isn't perfect, only less wrong than one datatable.

 

[Raptor]

Thanks Sierra22 your the first one I know to have actually tested it and shows it makes a difference!!! I have been out shooting when a front moves in and I can tell the difference in my bullet impact on days like that also. This has been the only time I have actually seen it change in the same shooting session.

 

[sierra22]

Raptor, I probably don't see as large changes during the same shooting sessions as you can due to pressure - my local weather simply doesn't change fast enough.
I lived almost 2 yrs in North Dakota, so I think I know what a front means to you. I remember jogging in sun and almost swimming & dodging flying debris before reaching home. There's simply a lot more going on in the Dakotan sky.

I can probably get away with looking at pressure once and use the same for several days - the exception being during storm fronts. Temperature can rise/drop sharply morning and evening but is fairly stable 11-16. The wind can do anything, and where I shoot topography really can give you a beating; severety depending on wind direction.

The pressure-effect-thing discussion has been put to death over here. Measuring and correcting for pressure is now fully integrated sniper doctrine, especially for the big guns. Basically it's artillery math applied to rifles; the end user product is the data tables. I don't understand the math, but I can certainly correlate weather station/data table. Privately I use the computer.

I rarely get more than 4-5 clicks deviation in elevation, that's impact from collar to belt on the silhouette with center aim. I often find that temperature and pressure work in opposite directions, making the total correction smaller. More common is 1 or 2 clicks.

When I started all this wasn't in the drills - calling the shots was a lot more :, What I'm experiencing now is that the call is a lot firmer with mostly horisontal deviations.

 

[Raptor]

Sierra22,
Your close to what I was experiencing. It was usually about a .5moa at 500 or 600 and then about a full MOA on out to 1000yds. Where are you located??

 

[sierra22]

Norway.

Was out shooting today, and from early morning till dark I had a change from 949 to 951 hPa. Over the next couple of days it'll probably approach 960 if the trend continues. These changes won't influence on my comeups.

However, pressure 949hPa was a significant deviation from when I first shot my "baseline", at 996hPa. At 600m 2 clicks less, at 800m 3 clicks less. Temperature was 5C now vs 4C then. I had 10-14 clicks left windage on the sight today, so I don't fully know if all of it is due pressure. I didn't have the presence of mind to record windage when I shot the baseline the first time.
I'll have to run the numbers through the computer later and do a little guessing and see how well it correlates to my datatable.

As I said, the system isn't perfect. It's sort of manual PDA with reality checks.
---
just did :,, and I guess I'm in for another shooting session next week ,edit: right input in the program works too...)
---

And today monday the pressure has reached 956 hPa. That's up only 7 hPa from friday, which is insignificant.

 

[sierra22]

Raptor, you should pick up a copy of Precision Shooting september 2006. Just got an article reg bullet performance/ air pressure mailed me.,about the DTAC reticle by Tubb/Cole)

 

[rob223]

spin drift: 1k-2moa, 900 1moa, 800 .5 moa