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Old 23rd January 2010, 06:45 PM
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RobF RobF is offline
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Originally Posted by Sniperdad View Post
Hi all, I'm new here, My name is Dave from South Africa.
Started Ft in Feb last year and had good results. But this Point of impact shift thing had me up the wall sometimes. If I lost a comp it was as a result of one or 2 shots not going where I aimed, often low, but sometimes high.

Last year I shot with a Walther Dominator LG300 with a Big Nikko scope, which was giving me shifts but I must say on the modest side. But they were shifts none the less and resulted in silly misses.
This year I have been sponsored my dreamgun, an EV2mk4.. I still have the same scope on. Its nicely accurate but also shifts ,( like my other guns).

I only recently came back from leave (Sun City) where it is a scorching ave of 36 deg cels daily(he he he ) so me and the EV has only had 3 weeks or so to get to know each other, but I noticed she shifts aswell. Especially during turbulent thunderstormy weather or cloudy days where the sun comes and goes.

I get and instantanious 3-5 click shift when cloud cover moves to sunny and vice versa, at the same time I can feel the temp drop or climb, sometimes it shifts and returns back within 20 minutes. It plays havoc with a score card.

It seems to be universal here with us in South Africa as I have not encountered a gun that doesnt shift. The Question is simple, as you guys have vast experience, "Is there anything you can recommend to reduce the effect of temp shifts on my EV". Or is the sollution not mechanical but rather awareness of atmospheric conditions?

Hi Dave,

I've been trying to get to the bottom of shifts since I started.

This is as far as i've got... there's 4 types of shift...

1) Mechanical... ie the gun bending, the scope mounts expanding, basically your contraction of metals or things warping... i've heard guns even doing it as the cylinder pressure changes and changes the stress on the action... float barrel.

2) Speed shift... ie the speed of the pellet changing between hot and cold due to the way the gun handles it... tricky because I can really only chrono at certain times, and even then, the speed changes that are found are small, and shouldn't be enough to cause the shift seen, however, they could combine with cause 3.

3) Ambient conditions shift... ie thinner or thicker air (like we found in SA, with a flatter trajectory at 4000ft)

4) Rangefinding shift... caused by light/bright or dark targets causing a change in the eye's pecption of focus, or where the range is actually focused. As the depth of field increases at longer ranges, the eye can be tricked into picking up an area in the focus range it doesn't normally work on. So some people come in to focus and use that, some try and push it out, some pick the best... but the light can change what the eye sees.

To expand on scopes, it could be that you get a mechanical shift due to the expansion or contraction of the scope or it's parts itself which mean it actually range finds a different range on a known target range in constant conditions.

I haven't ruled out that the body or the perhaps a metal ret, or perhaps the thread which changes the ret's position could change in temp perhaps...

I wonder how much does the ret actually move in mm to move 1/8th MOA ?

Speed changes can be found with a chrono, but thinner or thicker air can change the trajectory... in SA i was 4 clicks high at 55, yet spot on at 30 and the speed was the same... she was flying flatter in the thinner air, more than chairgun would suggest... i think most of the team saw it.

The only lateral shifts i've seen have been up on that hill, in 40+ degrees, and i'm not ruling out that the gun/scope was bending/warping from being lain on one side... i've certainly not seen anything like the movement I had at close ranges sideways in any other normal conditons, and never lost so many to reducers, despite being in far worse wind.

The fast fluctuations on the walther were solved by changing the valve seat to a harder material... the green seals seemed too soft, and a change by a degree would = a click within a few minutes. Real trouble on courses with shaded cool areas and hot exposed areas... that's solved. Harder seal, much more stable. Even with soft PTFE I didn't see a single click of shift all across last summer, however it moved when the temp dropped 10 degrees on the first winter shoot by 7 clicks (1/4 MOA). After that it was stable again.

Nowadays I just zero before a comp. If there's 1-2 clicks difference, or even 8 i just add/subtract them from the entire ranges and go with it. If it's 1-2 i wont bother doing new numbers, if it's 8 I will.

My walther hasn't shifted since coming back from SA. However it didn't match the numbers I went out with. I've become accustomed to just perhaps seeing a big shift say between winter and summer, that doesn't shock me. I might see 1-2 on a cold or hot day away from the norm, but whatever I dial in seems to hold for a while, unless there's a dramatic change in weather.

I guess the work goes on dialing out mechanical shifts, and learning how to mitigate the effect of light on the scope, and just dealing with ambient shifts out of your control. Having a scope that you're 100% confident with really helps... my custom shops don't change range under temp (if they do I don't notice it)... that said they struggled with the very bright conditions down across the dead wood in SA... and they seem to do so here, but if I can find a fine bit of detail to work on, it really helps dial out the error... trouble was there, there was none... floating targets in mid air, bright, no string frays. Last thing i'll range on is the target itself... play with some white/black/coloured targets and find out why

Personally, I think if you see shift, even a small amount, you're probably in a better situation than blindly missing until you've shifted so far you start missing targets completely with no chance of hitting them.

A cold course will have shots missed low, hot normally high.

I don't buy the air pushing down/up. I've only seen one target where I couldn't explain a high miss group, and another where i've seen it high a lot I reckon is due to them being low targets shot low across hot ground on a colder air day... ie i think the light was bending. At 40 yds the group was so high people would have to have been ranging 5-10 yds out...unlikely considering the company. Well that's my excuse and i'm sticking to it

I suppose at the end of the day, it doesn't matter whether the theory is right or wrong, it's recognising it and the action you take.
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Last edited by Robf; 23rd January 2010 at 06:47 PM.
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