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Goofy question 300gr berger/spin drift

 
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  #1  
Old 01-15-2012, 09:32 PM
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Goofy question 300gr berger/spin drift

I have noticed something strange the last two range sessions. Been tweeking an Armalite AR 30 338 lapua. Settled on 91 g H1000/300 Berger Hybird. Loaded to fit mag at 3.72 (mag lengh is 3.75) Anyway, the last two days out I have had a breeze from 3:00. Today I confermed my zero at 300, and shot 2 shots at 400 - 800 yards, every 100 yards...just to conferm drops. I was dead on at 300 for windage. 400 not to bad for windage, but as I got out to 800 I was hitting 3/4 to 1MOA right..with a R-L wind....maybe 3-5MPH. I should have been left 1MOA. I could see the slight mirage off of the snow....definatly R-L, but I just kept going further and further right. I went back and shot 300 after and was again dead on for windage.

I am shooting almost due east at the 49th paralell for anyone who has the abilaty to figure coriolis...I don't. I can't imagin that any of that stuff (spin drift and coriolis) would have any noticable affect at only 800. This is my range...I shoot it all the time...never seen anything like it before.

Anyway, my drops are also off, unless my chrono is way off....I am needing to tweek the speed up and the BC up. I am having to add 75 FPS and put the G1 BC on the I6 program to .850.

Gun is shooting good, but NOTHING is going according to plan.

Has anyone notice that the 300 grain Bergers DO NOT pay any attention to the laws of phisics???


EDIT......I was just kicking back doing nothing gun related and had an idea....scope/rifle cant. Now, I mounted the scope and am usually pretty carefull. As far as the shooting, I am shooting off of a Harris bipod that tilts...but I cant imagin I am that far off. I am thinking that impacts would also be low, not high.

Last edited by 4xforfun; 01-15-2012 at 10:02 PM.
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  #2  
Old 01-15-2012, 10:00 PM
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Re: Goofy question 300gr berger/spin drift

Sounds like more of a canted scope issue to me. Are you using a anti cant level?

CD and SD should not be this big at 800. I do get about .5 moa of SD at 1000 with mine.

You mentioned wind, was this extra drift in addition to your wind correction?

Also, I can not speakfor your velocity, as I am not sure on barrel length but it should be in the 2700~2750 ish range with that load??

As for that .850 G1 BC, toss it. They are not that high and I have done a lot of testing with the Gen2 Berger OTM 300's.

Jeff
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Old 01-15-2012, 10:57 PM
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Re: Goofy question 300gr berger/spin drift

If you just mounted a new untested scope you may find that your erector tube does not move in a perfectly vertical line when you dial in your dope. You can test this at 100 yards by hanging a weighted string in front of a 4' tall piece of butcher paper. Place a dot at the bottom and to the side of the string and shoot it. Dial your scope up to your 400yd dope and shoot again ( approximate 25"). Dial up your 500yd dope and fire again (approximate 46"). All your shots should measure the same distance away from the string. If they start to walk away from the string either your scope is canted or the erector tube doesn't track straight up and down. Either way it is an easy fix to rotate your scope until the above test tracks perfect. I actually know some guys that rotate their scopes to the left to compensate for spindrift so their windage drop charts are neutral at longer ranges as sort of a compromise. If your not running a level on your rifle you need to. Without a level you can cant the rifle enough to throw shots 8-10 at 800yds. Just a few things to mull over. I show my 338 ultra as having 1.6" due to coriolis with the figures you stated. I also show 3.4" for spindrift at the same range.
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Old 01-15-2012, 10:57 PM
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Re: Goofy question 300gr berger/spin drift

Quote:
Originally Posted by Broz View Post
Sounds like more of a canted scope issue to me. Are you using a anti cant level?

CD and SD should not be this big at 800. I do get about .5 moa of SD at 1000 with mine.

You mentioned wind, was this extra drift in addition to your wind correction?

Also, I can not speakfor your velocity, as I am not sure on barrel length but it should be in the 2700~2750 ish range with that load??

As for that .850 G1 BC, toss it. They are not that high and I have done a lot of testing with the Gen2 Berger OTM 300's.

Jeff
I just checked the scope/ rifle with a buble level...perfect. I did notice that the muzzle brake is canted...one of those huge arrowhead brakes. Fixed that. Anyone know if the Ellen screws are suppose to be on the bottom or the top...these are on the top. I didn't want to change that, as that would change the harmonics...I would need to re-tune the load.

Wind......No, I am actually BUCKING the wind....My impacts should be left of target, I am hitting to the right....into the wind.

Velocity.....My chrono told me 2675......I had to bump that to 2750 and tweek the BC to .850 to make my zeros work out. I havn't gotten way out there yet. I am going to work out to 1200...that is as far as I can go on my range. I am going to be extra aware of the canting issue next time out, but the scope/rifle check out ok.
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  #5  
Old 01-15-2012, 11:11 PM
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Re: Goofy question 300gr berger/spin drift

I really like to shoot the rifle to cnfirm the scope is tracking true to the level. The above post is one way to do this. If you are cranking up turrets and the reticle path is not true to the anti-cant level you will track off in windage and it will get worse as the shot distance increases.

Jeff
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Old 01-15-2012, 11:19 PM
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Re: Goofy question 300gr berger/spin drift

Also, I see you have a 300 yard zero. I have been that route. I did this when using a 8x32 NXS as I wanted all the elevation I could get. So I custom milled a base to be only a moa off bottom when I was zeroed at 300. This was a lot of work that didnt gain me much. 300 yrd zeros can have some issues. For one it is harder to check as wind can affect it. Plus group sizes are bigger and can toss you off 1/2 to 1 moa or even more. This could be some of your program solution issues. I now have all mine zeroed at 200, but 100 is probably the best choices for all intended purposes. I use 200 because my 338 with 300 will shoot tighter and more consistant at 200 that it does at 100. Even though some people will ague that is impossible it is true. After close to 1400 rounds with this rifle I have seen it enough to not argue with it.

So, I feel it is much better to zero at 200 or 100 and if you want a point blank hold for 300 dialit in before you hunt.

Jeff
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  #7  
Old 01-16-2012, 02:06 PM
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Re: Goofy question 300gr berger/spin drift

Quote:
Originally Posted by Broz View Post
Also, I see you have a 300 yard zero. I have been that route. I did this when using a 8x32 NXS as I wanted all the elevation I could get. So I custom milled a base to be only a moa off bottom when I was zeroed at 300. This was a lot of work that didnt gain me much. 300 yrd zeros can have some issues. For one it is harder to check as wind can affect it. Plus group sizes are bigger and can toss you off 1/2 to 1 moa or even more. This could be some of your program solution issues. I now have all mine zeroed at 200, but 100 is probably the best choices for all intended purposes. I use 200 because my 338 with 300 will shoot tighter and more consistant at 200 that it does at 100. Even though some people will ague that is impossible it is true. After close to 1400 rounds with this rifle I have seen it enough to not argue with it.

So, I feel it is much better to zero at 200 or 100 and if you want a point blank hold for 300 dialit in before you hunt.

Jeff
I get my windage zero from 100 yard on a zero wind day (or night) and go up from there. Now, I have not checked weather or not the scope tracks strait up and down.....we just put on a 5.5-25x56...have not checked it yet. That is simple enough, and I will do it first thing next time out.
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