(Help) Verifying a 100 Meter (Z) at 25 meters with a 150 gr .308
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Thread: (Help) Verifying a 100 Meter (Z) at 25 meters with a 150 gr .308

  1. #1
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    (Help) Verifying a 100 Meter (Z) at 25 meters with a 150 gr .308

    I zeroed my 20" barreled Rem 700 SPS Tac .308 @ 100 meters with a 150 gr Rem express PSP round with a BC .314.
    My range data after shooting it came out to:

    Range in Meters Drop Velocity
    0-Muz 0.0 2820
    100 0.0 2541
    200 -1.7 2278
    300 -4.5 2031
    400 -7.0 1800
    500 -11.9 1589
    600 -16.9 1402

    This was done 6 months ago, I haven't touched my rifle after I zeroed it except clean it before I picked it up for deer season. I don't have access to 100 Meters any more, however I have access to 25 meters.

    If I am to verify my 100 meter (Z) at 25 meters I need to shoot 4" below center of target?

    If I am to verify my 100 meter (Z) at 50 meters I need to shoot 2" below center of target?

    Is this correct?
    V/R
    Chris

  2. #2
    Senior Member Winxp_Man's Avatar
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    So this ammo fps might be calculated in a 24" barrel. If it is then a 20" barrel will probably yield about 2740 fps. So at 2740 at 100 yards your looking at a 2.62" drop if you have a 25 yard zero. So when all is said and done you need a 2.62" up on a 25 yard target to hit a bullseye or close it it at 100 yards.

    We have to remember bullets starts dropping the minute it leaves the barrel almost. Gravity has a way to do that to us or else we would be floating around or have had the ability to fly haha!
    Last edited by Winxp_Man; 08-31-2015 at 01:37 AM.
    shoot to kill not wound !

    Firearm Pimp

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    Everything I have put up in the OP was with a chrono out of my gun at 100 meters. So your saying i would need to shoot 2.63" below the center point of the TGT at 25 meters to check my 100 meter (Z)?
    V/R
    Chris

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    Senior Member Deadshot2's Avatar
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    Just remember that the scope's "Bore Height" will be important when doing this exercise. If your scope center is 1.5" above the bore centerline you'll have entirely different numbers than if it is 2+" above. Especially at 25 yards.

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    There are 4 issues developing in this thread that need to be addressed.

    1. Sight height is a vital piece of information and you can't accurately determine a 25m POI with a 100m zero without it.

    2. Your specified range numbers don't make sense if the range is in meters. They do line up fairly accurately with the trajectory for that round at an MV of 2820 fps with a 1.9" sight height. To get those numbers at the 9% longer range in meters, you'd need significantly more velocity and that's not going to happen in a 20" barrel where the velocity will be a bit less. Depending on the load, the difference can be as little as 25 fps or as much as 150 fps with a 20" .308.

    3. You can't take the drop at 100m that you get with a 25m zero, come up that amount in inches at 25m and expect to have a 100m zero. I calculate 3.3" of drop at 100m (3 MOA) with a 25m zero and a 1.9" sight height using the OP's load and MV. If I were to come up 3.3" at 25m, I'd be zeroed at about 500m.

    I can however come up 3 MOA at 25m and get an approximate 100m zero. An angle of 3 MOA subtends a linear distance of .856" at 25m which (surprise, surprise) is how high you'd need to be impacting the target at 25m to be approximately zeroed at 100m.

    4. Using a 25m target to create a 100m zero will at best only get you in the ball park. A difference of 1/10" at 25m will make a big difference at 100m. Similarly, 1/4 MOA adjustments at 100m will equal .285", but at 25m, that same 1/4 MOA click will result in a change of just .071". Consequently, a minor variation in bullet impact, or a minor error in determining the center of the impacts can leave you a couple clicks off in both elevation and windage. Then, if you plan to use that basic 100m zero to add elevation and windage on the scope, you will be magnifying that error significantly as the range increases.

    -------


    Consequently, if you are thinking that you can use published load data and a 25m range to develop an accurate 100m zero and then go hunting at ranges in excess of 100m, you are not a road to either success or responsible hunting.

    You need to find a 100 yard or meter range and develop a proper zero for your rifle, particularly if you plan to add dope on the scope for shooting at longer ranges.

    Ideally, you'd also have actual chrongroph data for your load in your rifle, and/or true the predicted ballistics with actual results on target at long range.


    ------

    One final thought. If you were raised using imperial units (GA address) and are comfortable estimating range in yards and target size and hold over distances in inches, feel free to just use yards and inches. There's no need to use meters just because it's common with military trained shooters, and there is no advantage to mixing imperial and metric units - it just makes the math harder.
    Last edited by Model 52; 08-31-2015 at 09:13 AM.
    Stang likes this.

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    "Consequently, if you are thinking that you can use published load data and a 25m range to develop an accurate 100m zero and then go hunting at ranges in excess of 100m, you are not a road to either success or responsible hunting".

    Thanks for the comment about the responsible hunting. I am on my buddies property that has a lot of coyotes terrorizing his cattle. I wanted this to get close to center mass so tomorrow I can confirm at 100 meters.


    "One final thought. If you were raised using imperial units (GA address) and are comfortable estimating range in yards and target size and hold over distances in inches, feel free to just use yards and inches. There's no need to use meters just because it's common with military trained shooters, and there is no advantage to mixing imperial and metric units - it just makes the math harder".

    I use meters because thats what I know and have grown up with in the Army. I dont talk in feet or yards, I talk in meters. I dont shoot a lot at mid-long range is what I asked about. I shoot tons of 5.56 (M4 carbine), to you may be an AR 15 with .223. Anyways thanks for the help and responsible hunting tips.
    V/R
    Chris

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    Senior Member rkittine's Avatar
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    Although new to tactical shooting, I have been shooting long range and reloading for years. Looking at the line of sight of the scope and its height above the barrel, the projectile is not dropping when it leaves the barrel as it is being Lobed so to speak and is rising. Many loads will produce dead on at 25 yards on the upcurve and dead on again at somewhere around 100 yards as it starts to drop, depending on velocity at around 100 yards and then continued drop beyond that. Knowing the velocity at different yardages, you could calculate the drop from a barrel that was parallel to the ground but again, the bore will have a slight rise upward as compared to the line of sight of the scope. Put up a substantial sized target at 25 yards and see.

    Bob
    Robert Kittine
    Sag Harbor and Manhattan, New York

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    Quote Originally Posted by cmr287 View Post
    "Consequently, if you are thinking that you can use published load data and a 25m range to develop an accurate 100m zero and then go hunting at ranges in excess of 100m, you are not a road to either success or responsible hunting".

    Thanks for the comment about the responsible hunting. I am on my buddies property that has a lot of coyotes terrorizing his cattle. I wanted this to get close to center mass so tomorrow I can confirm at 100 meters.


    "One final thought. If you were raised using imperial units (GA address) and are comfortable estimating range in yards and target size and hold over distances in inches, feel free to just use yards and inches. There's no need to use meters just because it's common with military trained shooters, and there is no advantage to mixing imperial and metric units - it just makes the math harder".

    I use meters because thats what I know and have grown up with in the Army. I dont talk in feet or yards, I talk in meters. I dont shoot a lot at mid-long range is what I asked about. I shoot tons of 5.56 (M4 carbine), to you may be an AR 15 with .223. Anyways thanks for the help and responsible hunting tips.
    You may be talking in meters, but your range data looks like it's in yards, and based on published ballistics not actual shots on target. You need to true that up with shots on target at those ranges.

    As far as confirming the zero at 100 yards, that's certainly the thing to do, you just didn't mention it in your original post.

    Personally, I've found that with a bolt action rifle you can place the rifle on a rest or on a bipod and rear bag and then visually bore sight the rifle at a target at 100 yards or meters and be on the paper with the first shot. If you've got a mil dot or MOA reticle to measure the distance from center, the second shot will be on the X and the third round will be confirming the zero.

    That'll save you the time on the 25m range.

  10. #9
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    I use app STRELOKPRO and it would get you very close as long as you have entered the data correctly. If you have already some points of reference it will be even closer e. g. You have some records of the actual calibrations vs poi.

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    Quote Originally Posted by cmr287 View Post
    I zeroed my 20" barreled Rem 700 SPS Tac .308 @ 100 meters with a 150 gr Rem express PSP round with a BC .314.
    My range data after shooting it came out to:

    Range in Meters Drop Velocity
    0-Muz 0.0 2820
    100 0.0 2541
    200 -1.7 2278
    300 -4.5 2031
    400 -7.0 1800
    500 -11.9 1589
    600 -16.9 1402

    This was done 6 months ago, I haven't touched my rifle after I zeroed it except clean it before I picked it up for deer season. I don't have access to 100 Meters any more, however I have access to 25 meters.

    If I am to verify my 100 meter (Z) at 25 meters I need to shoot 4" below center of target?

    If I am to verify my 100 meter (Z) at 50 meters I need to shoot 2" below center of target?

    Is this correct?
    I didn't read through all the answers yet, so I will grab this one for you and apologize if someone else already did.

    For a bullet to hit a specific location at 100 yards, the barrel has to be appropriately pointed so that the bullet drops the correct amount from its maximum apogee. As such, if you say your "zero" is 100 yards, the bullet would have been higher from the center of the earth from the time it left the barrel until it hit that point. Just basic physics and the laws of gravity. Now, to get granular to your question - the flight of a bullet has a lot of characteristics. These include air density / altitude, temperature, relative humidity, wind speed and direction, ballistic coefficient of the projectile, and distance/direction from north pole with regard to Earth's axis. In the Air Force we called that the firing solution.

    At any rate, when you are shooting you are not looking through the barrel, which is how a computerized fire control system estimates projectile movement in larger weapons. So you also care about how high above the center of the barrel your eye (and scope or open irons) are. So there is the most wonderful piece of software on the planet available to you for your droid phone, your ipad, or your droid tablet. The name of the product is called "Shooter" and is under $10. You load in ever drop of information into this software (you can save the profiles of every weapon and every cartridge you own and it has a huge database); and it tells you exactly to the hundredth of an inch where the bullet is at any distance or time-of-flight. It pops up a spreadsheet that you can fine tune to show every 5 yards, every 10 yards, etc. This software will let you put in your altitude, direction from true north, temperature, humidity, etc. or will estimate averages. To know my altitude, pitch, acceleration, lat/long, direction etc. I use a piece of software called GPS Status; which also lets you calibrate for altitude (there are physical reasons why you would want to do that).

    BTW - I don't know where Harris County is but I lived in Paulding County for a decade and you are allowed to shoot at Fort Benning on specific days and that goes out to 1,000.
    Last edited by Snappo; 09-24-2015 at 07:02 PM.


 
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