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Thread: 100 yd sighting-in limitations

  1. #1
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    100 yd sighting-in limitations

    Recently overheard this:

    Sighting-in a high caliber rifle (.30-06, 270, etc.) to hit dead on @ 25 yds whereby the bullet is 'sposed to hit 1" to 1.5" high @ 100 yds....dead on again @ 200 yds...?
    Allegedly a technique to acquire reasonable bullet placement when a 100 yd range is unavailable.

    Can any of you fine fellas comment thereon?
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    Might want to download this free, educational software from Remington? It's a lite version of Shoot software?
    I'm mostly a pistol guy, but have recently gotten into ballistics a bit more rather than just shooting 'til I get my AR's on paper.

    The software's a great educational tool, and is easy to use/play with-shows POI on the target for the variables you put in. Can also give you printouts of whatever chart you want.

    I've downloaded the free Point Blank software also, but the Remington's kinda nice if you just want an educational tool.

    http://www.remington.com/products/am...s_software.asp

    EDIT: Just played with the software for a 30-06 and if you zero it at 100 yards looks like you're going to have a pretty wild trajectory path (Zero @ 25 and 600 yards, hitting a max high of 28 inches at 350 yards). If you zero at 100 yards, you're pretty close out to about 300 yards before it starts dropping rapidly.
    I'm no expert, and this is the first time I've plugged anything but a 223/5.56 into the software. Looks to me like on a cartridge like you're talking about, you'd want to zero it at a much greater distance than 25 yards to avoid those wild trajectory swings?
    'Course, this is inputting 3.5 inch scope height and using a 55gr. bullet.

    I'm sure the experts can set you straight, but playing with the free software can be worth 10,000 words?
    BTW...software shows 9.61 inches high at 100 yards with a 25-yard zero!
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  3. #3
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    Wouldn't it vary considerably based upon the height of the optical plane above the line of the bore?
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  4. #4
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    Wouldn't it vary considerably based upon the height of the optical plane above the line of the bore?
    Yes it would, but as stated, the figures I gave him are for a scope 3.5 inches above the barrel (which is the height of all my AR scopes).
    I went back and plugged in different scope heights and you can really flatten out the path the closer you get to bore center.
    'Course, we have no way of knowing what his scope height is, and what he heard at his gun shop doesn't cover his particular variables?

    Bottom line...he needs to get the free software and play with it!
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  5. #5
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    For your 30.06.

    Zero (point of aim + point of impact) at 25 yards is going to be "very close" to your 200 yard zero.

    The bullet is traveling up at 25 yards, on the way down at 200, they just happen to cross the same plane (line of sight) at those ranges. (It is closer to 250-300 yards for 223, and I suspect is the case with 270 as well, since drop is a function of speed)

    This is a "rough starting point" and you will want to confirm on the range, at actual distance.

    So, if you zero at 25 yards, you have near your 200 yard sight setting. If you shoot at a target at 100 yards, you will be near 2" high. Again, 'rough starting point".

    Darn good enough to get a white tail though
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  6. #6
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    Just for the heckuvit, I changed the scope heights since I assume most hunting rifles don’t have a 3.5” scope height as my AR’s. I come up with these figures for a 30-06:
    All ZEROED AT 25 YARDS
    1” Scope Height
    Zero @ 25 and 270 yards (2” high @ 200 yards and dropping, hitting a max of 3” high at 150 yards)
    1.5” Scope Height
    Zero @ 25 and 365 yards (6” high @ 200 yards-6” looks like max rise, as it starts dropping after 200 yards)
    2.5” Scope Height
    Zero @ 25 and 515 yards (14” high @ 200 yards and rising, hitting a max of 16” high at 300 yards)
    3.5” Scope Height
    Zero @ 25 and 602 yards (20” high @ 200 yards and rising, hitting a max of 28” high at 360 yards)

    Scope height makes a helluva difference!
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  7. #7
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    I was at the range today, started @ 25 w/ 30-06. I was dead on @ 25, moved to the 100 and was about 5 inches high. 165 Remmington Core Lokt. adjusted down to 2 1/2 inch high @ 100.

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    Jest playing here…I plugged your Remington ammo into my Remington Shoot software and came up with the following:
    I’m guessing your scope height is 2.3”, since that’s what it’ll take to shoot 5.1 inches high @ 100 yards using a 25 yard zero.
    If you adjusted down to 2 ½” high at 100 yards, here’s what I show on the bullet trajectory chart for your ammo:
    Zero would be @ 36 and 240 yards-looks like you’d hit a max height of 4” at 150 yards before bullet starts dropping.

    ‘Course, there are still variables-I can’t find anywhere in the lite version of the software to account for/change barrel length, but don’t know how much difference that would make.

    Jest curious-do you know what your scope height is?
    I'd be interested to know how close I came muddling through the software?
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    Interesting, Savage 110. Leupold base, rings & VXII 3-9x40.

  10. #10
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    You can also download this free:
    http://sourceforge.net/projects/balcomp
    (works on multiple platforms, including the Mac I use most)

    or use this on-line calculator:
    http://nobal.webdiligence.ca/

    Saved me a lot of time and ammo on my last range trip. Sighting in a .308 AR with a 3.125" sight height and an AR-15 (.223) iron sight BDC. Otherwise I would have been guessing....

    For the .30, you probably do want to choose a long zero (>200 yd) and just figure where that hits at 25, 50, or 100 (how high or how low) and sight in to that. If you have adjustable sights, leave yourself some room to dial down to hit between your near zero and far zero (just like the standard way to sight an M14).

    Hope this helps,
    David

  11. #11
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    Three and a half inches is pretty tall. The Aimpoint on my SIG is running about 2.25 inches from the center of the bore to the center of the optic.
    "Training errors are recorded on paper. Tactical errors are etched in stone." -Erwin Rommel
    Explanations exist: they have existed for all times, for there is always an easy solution to every problem — neat, plausible and wrong. - H.L. Mencken

  12. #12
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    Three and a half inches is pretty tall.
    It is-I've got a couple of AR's with scopes on top of the handles (ACOG TA01 and old Colt 3x Sporter)-3.5" didn't surprise me too much there, but when I measured my EOTech 557 on a flatop, darn if it didn't measure 3.5" also (I had to do a double-take/triple measurement on that one).

    Iff'n my EOTech wasn't boxed up and ready to ship for warranty repair, I'd measure that sucker again-I still can't believe it was 3.5" high.
    'Course, I'm just using scope height to plug into the software and play with-I'm just an old paper on target kinda guy, but the bullet trajectory graphs are eye-openers.
    Well, I did finally break down and buy a LaserLyte Deluxe Laser Boresighter since it can take a bit of ammo to zero all those toys hung off 'em!
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  13. #13
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    If your sight is co-witnessed with your irons on an AR-15, it should be 2.6", according to this:

    http://groups.msn.com/TheMarylandAR1...toverbore.msnw

  14. #14
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    If your sight is co-witnessed with your irons on an AR-15, it should be 2.6", according to this:
    My sights aren't co-witnessed-my EOTech has a 4X Magnifier, so the holographic sight isn't mounted on the handguards, which puts it higher than the distances they give in their article (notice they say mounted on the handguards, not carrying handle). I'm guessing the 557 EOTech also might be taller than other EOTechs-I don't have the opportunity to compare 'em side by side-the 557 unit may be a little taller than some of the other units.

    My old (around 30 years or more) Colt SP1 has a fixed carrying handle, so ain't but one place to put the scope (Colt 3x Sporter). I set up my Colt 6920 the same way (although it has a removable handle) since they more closely match up in that configuration (got an ACOG on it). When I measured the EOTech on my RRA, I thought I'd have to run different figures, but darn if it didn't measure the same height as both of my other AR's.

    Surprising, but cool? as all three should be pretty close to the same AFA scope height and ballistics.

    BTW…the new Colt 6920 and RRA sound pretty close to the same, but that old Colt SP1 definitely has a different sound to it! Doesn’t sound as “tight” as the other two, but it shoots just as well or better. Sucker’s so old it’s got no forward assist and has the old triangular handguards on it.

    Anybody wanna buy a “classic”?
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  15. #15
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    I see the idea of setting the scope at 25 yards pop up quite often.

    The question is usually asked like "Can I do it?"

    The answer is yes. You can calculate your trajectory and set it at 25 yards.

    The question that's usually NOT asked is "Do I want to sight my rifle this way?"

    The answer to this question is a resounding "NO!"

    Because of the way distance amplifies error, your 25 yard zero might be several inches off at 300 yards.

    In fact, I would be pretty comfortable betting a ten dollar bill on it being more than just a couple inches off at three hundred when it's been zeroed at 25 yards. My experience shows that it probably won't even be on the target.

    If you think you might need to make a shot at 200 yards, zero at 200 yards.

    The game deserves it. You deserve it.

    It might not be convienient, but you will feel a lot better knowing for a fact that it will hit at 300 yards instead of thinking it MIGHT hit at 300 because it hit at 25.

  16. #16
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    Sighting in

    The idea is simple. The 'scope is above the barrel. Assume the 'scope is horizontal. For the bullet to pass through the line-of-sight it must be pointed slightly upwards. Therefore the bullet will pass through the LOS twice: once on the way up and once on the way back down. It turns out that for a wide range of ammunition, if the bullet passes through the LOS around 25 yards, on the way up, it will pass through the LOS on the way down around 100 yards. Exact distances vary with trajectory of the bullet and 'scope height, but that's the general idea. If you get the numbers right, from the appropriate software, you can sight in a rifle at the shorter distance, and it will be sighted in at the longer distance.
    http://www.thejusticecooperative.com

  17. #17
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    that would be pretty much true. don't have the luxery of a 200 yrd range on a ship, pretty standard to shoot 25 yrds on the flight deck of smaller ships with the m-14. if your grouping well at 25 yrds, up to 2 inches, dead nuts at 200 yrds with iron sights anyway
    Real Naval Gunners Hit Harder, and Penetrate Deeper, and just as accurate without Fire Control!

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