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Thread: 25 yard zero with 6.8 SPC

  1. #1
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    25 yard zero with 6.8 SPC

    Did a search and generally came up with minus 2.5 inches as an answer.

    I want to get a 100 yard zero on a 6.8 SPC 16 inch upper, but only have access to a 25 yard indoor range to do it at.

    Where should my POI versus POA be for a 100 yard zero? Ammo will be the Federal Fusion 115 grain SP most likely with potential for the Hornady 120 grain SST depending on what's available.

    This will eventually be used as a shorter range hunting upper and shot placement will be critical for sake of the game.

    Is the -2.5 accurate for the weight class?
    Experience is a cruel teacher, gives the exam first and then the lesson.

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    Quote Originally Posted by Grand58742 View Post

    This will eventually be used as a shorter range hunting upper and shot placement will be critical for sake of the game.
    200 yards all day long with that hornady. Did it last year a few times

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    Before you go hunting, confirm your zero, for the sake of the game.

    My calc is telling me that a 100y zero should be 1.5" low at 25y. This is dependent on your scope over bore height. I use a larue mount so its 2.7" for me. Unknown for you.

    There's plenty of public ranges on Wildlife Management Areas all across Oklahoma that you can go test your zero at.
    http://www.youtube.com/wormydog1724

    Quote Originally Posted by Iraqgunz
    This isn't "Ihuntcoyotes.com.

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    Quote Originally Posted by Wormydog1724 View Post
    Before you go hunting, confirm your zero, for the sake of the game.

    My calc is telling me that a 100y zero should be 1.5" low at 25y. This is dependent on your scope over bore height. I use a larue mount so its 2.7" for me. Unknown for you.

    There's plenty of public ranges on Wildlife Management Areas all across Oklahoma that you can go test your zero at.
    This, it's not fair to the game to hit them too low and let me get away.

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    Using the KAC BulletFlight BC I came up with following results (You should confirm my information):

    6.8mm SPC 120 GR SST

    Sight Height 2.5 inches, MV 2,460, BC 400. 100yard zero the POI at 25 yards would be -1.3 inches

    Sight Height 1.5 inches, MV 2,460, BC 400. 100yard zero the POI at 25 yards would be -.5 inches

    If you keep the 25 yard zero and your sight height is 1.5 inches, then from 25 yards to 200 yards the maximum rize is 2.1 inches at 100 yards and at 200 yards you are -1.1 inches.

    If you keep the 25 yard zero and your sight height is 2.5 inches, then from 25 yards to 200 yards the maximum rise is 6.3 inches at 100 yards and at 200 yards you are 5.8 inches. At 300 yards you are -1.7 inches.

    Sight height makes a lot of difference.

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    Didn't realize the sight height has that much impact.

    Thanks guys.
    Experience is a cruel teacher, gives the exam first and then the lesson.

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    Quote Originally Posted by Grand58742 View Post
    Didn't realize the sight height has that much impact.

    Thanks guys.
    Yeah, it's a big reason the short zeros like 25 yd have a very parabolic trajectory. BTW -I have always used 2.6 in as a sight height.

    Hornady and some of the other ammo manufacturers have good ballistics calculators on their sites. The info from the Federal site gives a BC of .378 and a muzzle velocity of 2470 fps. Plug that in with a 100 yd zero and 25 yd graduations and you will have your answer. I just did this for the Federal 115 round, as well as SSA 85 gr TSX ammo, but using a 50 yd zero, which I prefer on ARs.
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  8. #8
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    When I e-mailed LaRue and asked about the height over bore for the LT104 and LT139, 2.7" was their reply.
    http://www.youtube.com/wormydog1724

    Quote Originally Posted by Iraqgunz
    This isn't "Ihuntcoyotes.com.

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    What height is you optic above the bore?
    What velocity is your choice of round?

    In general, not accounting for the grains/BC of your bullet, or the change in BC at different velocities-

    If your not using a very low profile mount, or a high old school top of a carrying handle scope,
    your optic is probably about 2.5 inches above the bore.

    2000 fps loads (300 AAC, 7.62x39, heavy 30-30 bullets) tend to zero at 25/200 and be about 4 inches high at 100, drop like a rock after about 250. - but overall trajectory is pretty good compared to a 50 or 100 zero.

    2500 fps rounds (6.8, light 30-30, heavy .308, heavy 30-06, etc.) with a 25m zeros again about 285, goes out to about 325 before becoming a stone, but puts you 5-6 inches high from 100 to over 200. The 50/150 is a decent zero in this velocity, that is an inch high at 100 and pretty flat to about 250. A straight 100 zero will keep you pretty flat to a little over 200.

    3000 fps-ish, (5.56, 308, 30-06, heavy 300WM) give a 25/350-ish zero that is way high in between. The 50/200 is pretty decent out to about 300. A straight 100 to about 250. (I am talking decent trajectory for minute of combat/kill zone without adjusting mils- not that these rounds are not decent at doing stuff or being useful for longer ranges). Tickiling 3100 to 3200 fps does not really do anything for me at a 25 or 100 zero, but buys you about 25 more useful, flat trajectory range in the 50 zero.

    3500 fps (.225 Winchester, light 300WM) is pretty cool in my opinion. I think of it as super 5.56 and super 7.62. Its like a 50/300 zero with another 50 of pretty flat range.

    So, I know there are people that will say zero at 25/300, 50/200, a straight 100, etc. but it all depends and those numbers often apply to a different round than you are using.

    I say know what ranges are most important to you at a zero and how flat, what your velocity you will be using is, and base it off of that.

    Also, you mount plays a significant role. I know of no practical advantages to a luggage handle upper. Except one.

    If you are rocking a 5.56 old school carry handle scope (about 3.5 inches above the bore) a 50m zero gives you a sweet 0-350 trajectory that will put rounds in the box with no adjustments.

    A real low profile set up (1.5) will do the same to about 325 with a 25m zero.

    Anyways, beware any typos, and verify POI in real ranges.

  10. #10
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    Quote Originally Posted by Wormydog1724 View Post
    When I e-mailed LaRue and asked about the height over bore for the LT104 and LT139, 2.7" was their reply.
    2.7" is also what I came up with for my Aero mount when I measured.

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