Zero shift on travel to new location.

[Brown Dog]

Rifle zeroed.

Travel several hundred miles North (by train).

On arrival, check zero. Bullet MPI low: 2 x 1cm clicks 'up' required.

Successfully conduct hunting trip using comeup chart –but adding 2 clicks 'up' to all elevations.

Return home.

Check zero this afternoon. Rifle back to original zero (ie 2 additional clicks no longer required).

But….(and I know what you're all thinking)…..both the Northern zero and the home zero were conducted at roughly the same altitude (within 20m) at roughly the same temp (around 8 degrees C).

I can't explain this! /ubbthreads/images/graemlins/confused.gif

Any ideas?

 

[Dave King]

Brown Dog

This should generate a lot of discussion!!

Both Len and I have seen this and I still can't figure out what the cause might be. (It's definately NOT due to rough handling of the rifle). There is something going on and maybe we can understand it better collectively.

 

[LB]

I don't know about explaing it, but I can attest to the phenomena?

All it has learned(?) me, in <font color="red">XX </font> years, is to always check my zero upon arrival. LB

 

[Len Backus]

For the last three years when hunting in the west I have been checking my zero by firing at 500 yards. This way I am sure of the POI at the local altitude. By establishing this at 500 rather than at 100 yards, I also am confident I have adjusted properly for altitude and temperature. Potential drop table errors due to changing local conditions would then have to be awfully small if I am on at 500 yards. (My kills have all been under 700 yards)

 

[Brown Dog]

Len,

The 500m check to reduce errors at other distances is a good idea, however [and for the purposes of theoretical examination, assuming that the rifle has not been roughly mishandled] why, when conditions are apparently (as good as)identical –less a 600 mile move North- should the zero move? I can't explain it to myself!

My confusion is caused by the fact that the zero shift was discovered when fired under broadly identical conditions to those where I zero at home:

roughly the same alt and temp,

same bipod off same surface (grass/mud),

same rear rest,

Using a rifle built like a girder: its action is glued and bolted to the stock chassis and its barrel has almost enough free-float to get your hand around it! (a minor exaggeration!).

Why then, given that everything is apparently the same (within tolerance!) should the zero drop 2cm at 100m when I travel around 600miles North; only to return to zero when I travel S again?

The only major variable is the 600 mile location change.

It makes no sense !! /ubbthreads/images/graemlins/confused.gif

 

[Edmac]

I had the same experience when in Scotland this September. POI 2 clicks right. Got back home, checked zero, back to normal. Weird!!
Ed

 

[Brown Dog]

Season's Greetings to all!

No one got a pet theory for this that they're prepared to expound in public? /ubbthreads/images/graemlins/smile.gif

 

[Imortal Wombat]

Well, I'm not exactly an expert at this, but may I suggest that the POI change may be due to different humidity levels at the various locations?
If this doesn't explain the change perhaps there has been a change in the "angle of the dangle" /ubbthreads/images/graemlins/tongue.gif at the various locations? I have herd that used to explain many shooting related issues, perhaps it explains this as well.

 

[Shawn Carlock]

Brown Dog,

I have experienced this as well at times in elevation and others in windage. Most of the times were elevation related only. I believe that in at least one case I can chalk this up to unread conditions. Case in point: 260 Rem precision rifle shoots under 3/8 moa in good conditions all day long out to 500 yards or so. Zeroed for 100 yards, 54 degrees, 34% RH, 29.6 BAR, zero wind, and shot at 17:37 hrs. 16 days later, conditions: 100 yards, 51 degrees, 36% RH, 29.8 BAR, zero wind and shot at 09:42 hrs. POI shift of +.60". I thumbed through my shooting log to locate my zero conditions, when I located them I looked through my spotting scope at the group position and noticed vertical mirage that I had also seen through the NF scope I was shooting. I had atributed it to barrel heat on the rifle but it was obviously a vertical thermal with a no wind condition. Taking note of this I came back in the evening and shot with the POI perfectly centered. I believe that the reverse of this has happened to me where I have zeroed in the morning and shot low in the evening but could not find any shot log info to support it. It seems that most of the times and different rifles I have seen this with were only effected by 1/2-3/4 moa @ 100 Yards. I believe that this is about the max bullet movement for a windless vertical thermal. I hope to get the right conditions to test this theory by keeping my late afternoon zero and reading the morning heat up thermal and correcting for a first round centered hit.

 

[Brown Dog]

IW and Shawn,

Apologies for my delay in answering. IW -both zeroes were conducted with a zero degree angle of sight. Humidity? I can't measure that directly, however temp and pressure (which clearly take account of humidity) were broadly constant.
Shawn,
Fascinated by your 'vertical mirage' theory. Perhaps it highlights that a wind may have vertical vector components that we fail to recognise. For example although both zeroes were conducted with -say- a 10mph cross wind. The 1st locations topography or whatever may have caused the wind to have an -unnoticed- 2.5mph positive vertical component; the other location may have a 2.5mph negative vertical component....there's your 0.5MOA shift /ubbthreads/images/graemlins/shocked.gif!
Be interested to hear what you deduce from future data that you collect! /ubbthreads/images/graemlins/smile.gif

 

[Shawn Carlock]

BD,
I have noticed this effect on level ground but have seen extreame cases shooting down the length of a canyon. I was once shooting a 308 @ 932 yards down a canyon, around 10:00 hrs. The wind was a slight breeze in my face up the canyon maybe 2 or 3 mph. There were calms in the wind of still movement down the length of the canyon to my target as far as I could tell. Now instead of shooting lower in the 2-3 headwind I shot high of my drop chart correction. I made a correction down, to be zeroed in the 2-3 mph headwind. Now zeroed in the headwind and shooting in the calm I shot nearly 4 feet low. I could only explain this from the thermal updraft that was to me undetectable other than to know that at 10:00 hrs the air most days is warming up. This was in the summer a few years back and night time to day time temps would change 40-50 degrees. This condition would cause a serious updraft in a canyon and a higher bullet strike. I think it is conditions like this ( but to a much lesser degree ) that cause zeros to shift back and forth. Actually if you buy my somewhat backed up theory the zero never shifted. The conditions were not read correctly and allowed for. Once again human error on my part. I have learned to read canyon thermals fairly well, but it is just that a reading. I know of no way to accurately read these conditions. But like anything else, enough trigger time in the conditions and you get some what of a feel for them.

 

[goodgrouper]

Brown dog,
Sorry I didn't read this post earlier.
Yes, there is a reason for this phenomenon and it is not a theory at all. It is a scientific fact called gravity intensity variation (giv). It basically means that as you change your latitude (horizontal lines on a globe) north or south, the strength of gravity changes slightly due to many factors but mostly the centrifugal force of the earth's rotation. The closer to the poles you get, the more gravity intensifies. Scientists have documented this fact extensively over the centuries with the Early Greeks being the first to discover it. They observed free swinging pendulums showing a difference in how quickly plumb knocked over blocks lined up in a circle around the pendulum as they moved the experiment from one location to another. Gravity seemed to change on the plumb the farther north they got.
It gets pretty complicated because there are actually many factors that play into it, but if you are up to reading the exact scientific mumbo jumbo, go to
http://stommel.tamu.edu/~baum/reid/book1/book/node32.html
or get the book called GRAVITY ANOMALIES by Stommel.
The best thing to do to overcome this cosmic problem that plagues us shooters is just as the earlier poster said. Just rezero your gun anytime you go north or south of home!

PS. I think this phenomenon is also to blame for people all over the globe complaining that ballisitic programs and drop charts and cosine indicators aren't working like so and so says there supposed to! Also, it shows how Drag 1 computations might differ from what you actually see in the field. One thing I would love to get a hold of is a program for the Iceland military (if there is one) and see how far off their drag function forms are from the ones we use that were developed in the 1930's somewhere in New England. Being that far north, there should be a significant difference in drop and drag due to gravity intensity.

 

[goodgrouper]

Just opened the new Varmint Hunter (Jan 2005) and flipped to page 94 and there it was, a great explanation of this very topic by Art Pejsa. I personally can't stand the guy, but he wrote a great article. Check it out if your a member of the VHA!

 

[Brown Dog]

Goodgrouper,

Wow; just read your link. That'll be a part of the answer /ubbthreads/images/graemlins/smile.gif!

I would suspect that -with relatively small latitude changes- it is the gravitational anomalies that are the major player (ie denser rock and the Earth not being a perfect sphere.)But I think the effects would be tiny:

I moved just over 6 degrees N between zeroes; Baum's article (your link) indicated gravitational acceleration increasing from 978.9cm/sec/sec at the equator to 982.3 cm/sec/sec at the poles. (Which is interesting because the Brit Standard unit for gravity is 981cm/sec/sec –which about matches our latitude for Baum's values.)

Some very broad bush thinking: With Baum's values if all other conditions remained the same (pretty difficult to do!), a projectile with a 1 second time of flight would impact 3.4cm (1.3in) lower at the pole than the equator. So at 100m (0.13 sec TOF for my 308) we would see a 0.44cm (0.17in) change of impact between equator and pole –not too significant! –particularly when you consider that I only moved a little over 6 degrees N and would therefore see only a tiny fraction of that equator to pole change.

What, I suspect, would be more significant would be the gravitational anomaly side of life. I live on chalk downland (pretty low density) the Scottish Highlands are one huge lump of granite (pretty dense!).

However I still think that the values would be pretty tiny at 100m (probably unmeasurable?).

I think a stronger effect would have to be the cause of 0.5 to 0.75 MOA vertical zero shifts at 100m (such as Shawn's vertical wind component idea).

What do you think? (is my thinking flawed? -I did a year-long course with a significant ballistics component about 7 years ago, but it's all very rusty to me now!)?

I have no access to the Pejsa article. What sort of values does he reckon you would see?

And now that we've entered full-on 'ballistic nerd-itry' where's Dave King's input /ubbthreads/images/graemlins/grin.gif?