Affects of 20 MOA base

Affects of 20 MOA base

This is a discussion on Affects of 20 MOA base within the Technical Problems forums, part of the SniperForums.com & SniperCentral.com Related category; I have recently taken up long range shooting at a PRS facility, though I have been shooting since I was 7 since my father worked ...

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Thread: Affects of 20 MOA base

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    Pat
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    Affects of 20 MOA base

    I have recently taken up long range shooting at a PRS facility, though I have been shooting since I was 7 since my father worked at Olin - so not a beginner at shooting.

    I had an old Remington 700 308 varmint rifle with a Tasco scope, bought some Hornady match 155 amo - got Sierra ballistic software and headed to the range. Zeroed the rifle, grabbed my ballistic chart and started at 300 and shot out to 700 with settings on the chart, and made it out to 1000 with minor adjustments. My daughter thought that was really cool, so I gave her the rifle.

    I threw together another Remington 700. Started with a 308 SPS Varmint, threw in a Timney trigger, swapped out the stock for a McMillian and added a Harris bipod. For optics I found an old stock Weaver Tactical 4-20 x 50 on a 20 MOA base. Spent 2 months trying different loads to find one that would consistently shoot better than 1 MOA. Now my ballistic charts seem to be way off. It almost looks like the charted setting which is for 100 yds past the target is the setting I need. For example, my DOPE for 700 yds is what my chart shows for 800 yds.

    I've done the following:
    Chronographed the load - 2871 average SD of 10 for 10 shots. This is pretty consistent with SD's of 5-10 depending on number in the group.
    Loads are consistently sub MOA.
    Velocity falls right in where I'd expect based on load data.
    I've verified the chronograph with the Hornady 155 match ammo.
    I've checked the scope by hanging a 30" target with markings 2" apart vertically then setting the rifle 100 yds away on a bench and running it through 30 MOA of adjustment.
    I get roughly the same DOPE for the Hornady 155 ammo as my home brewed 155 TMK stuff.

    Now I'm left with 3 possibilities - my software is bad (which I cross checked with Hornady's free on line stuff), the 20 MOA base has caught me off-guard (I assumed all it would do is trade some of the bottom adjustment for top) or there is something I am overlooking.

    Any observations?

    Thanks

  2. #2
    Pat
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    How about some DOPE comparisons from some shooters

    Last trip out:
    155gr running 2850 fps
    Ranges: 350,400,450,500,550,600,700
    Calculated MOA: 4.8,6.1,7.6,9.1,10.7,12.3,14.9
    DOPE MOA: 7.3,9.5,10.8,12,14,15.5,21

    Anyone got some DOPE for 155 running approx 2800 for similar ranges?

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    Ballistic calculators aren't infallible they will give you a good idea of what to expect but the only want to be dead on is to dope your loads.
    I can't see how a canted base would effect any thing all it does it angle the scope down in the front a little.

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    Senior Member ddd oo7's Avatar
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    Ballistics calculators are not perfect...but they are usually very close. Thing is...dope changes with atmospheric conditions (temp, humidity, pressure, etc) so unless you have a dope chart for every condition, then the ballistics calculator is usually going to be more accurate. However...software depends on being fed accurate info.

    Actual shot on target should not be that far off of the calculator...something is wrong.

    Looks like you might have a G1/G7 mix up as far a BC of the bullet goes. Bullets have a G7 BC and a G1 BC. You must be sure that you are feeding the calculator the proper BC.
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    Senior Member tarheel7734's Avatar
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    If you are zeroed in with the 20 moa rail that affects nothing other than the amount of adjustment you have in the scope. If you are using different loads than your balistic chart was set up for of course it is going to be off. With each change of load you need new data. Its all about accurate data.
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    Senior Member mamalukino's Avatar
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    Chrono might be inaccurate.
    Back door the comeups into a ballistic calculator like the free hornady and you will get a more accurate MV.
    There are some BC out there that will let you input a your farthest shot to verify actual MV for that day.
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    Pat
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    Quote Originally Posted by Tim37 View Post
    Ballistic calculators aren't infallible they will give you a good idea of what to expect but the only want to be dead on is to dope your loads.
    I can't see how a canted base would effect any thing all it does it angle the scope down in the front a little.
    Me either - hence the post

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    Pat
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    Quote Originally Posted by ddd oo7 View Post
    Ballistics calculators are not perfect...but they are usually very close. Thing is...dope changes with atmospheric conditions (temp, humidity, pressure, etc) so unless you have a dope chart for every condition, then the ballistics calculator is usually going to be more accurate. However...software depends on being fed accurate info.

    Actual shot on target should not be that far off of the calculator...something is wrong.

    Looks like you might have a G1/G7 mix up as far a BC of the bullet goes. Bullets have a G7 BC and a G1 BC. You must be sure that you are feeding the calculator the proper BC.
    .02 MOA difference at 500 yds using G1 vs G7. I agree, something is very wrong - I have a can't see the trees despite the forest thing going on

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    Pat
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    Quote Originally Posted by mamalukino View Post
    Chrono might be inaccurate.
    Back door the comeups into a ballistic calculator like the free hornady and you will get a more accurate MV.
    There are some BC out there that will let you input a your farthest shot to verify actual MV for that day.
    Chrono speeds correlate with load data and match published speeds on the Horady 155 Match .... and those shoot close to my loads.
    I could just motor on with the dope, but I have a hard time walking away having no understanding of why I have the descrepancy

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    Pat
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    Ahhh - the stars align. I found the tree in the forest. There is a loose screw in the turret adjustment. If I pull all the way up on the turret I can turn and still get my clicks but the elevation dose not change. Since I was in my shop when I ran the adjustment check I guess I wasn't pulling as hard as when I was out on the range.

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