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Thread: Trouble with Mk18 zero

  1. #21
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    Quote Originally Posted by TylerD View Post
    As an update, got a bore sight and lined irons and I point up at roughly 20-25 yards. Do most zero SBR's at 50 or 100? Longer guns have a 100 yard zero but I'm debating what's best for this one.
    Since you're sorting this out for a carbine class, I would zero it for whatever the instructor recommends. Most have a preferred zero for their course but are flexible if you know how to get solid hits with your own preferred zero.

    Most courses start out verifying zero and making tweaks, but nobody wants to spend time chasing somebody's zero all over the target.

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  2. #22
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    Personally I'd disassemble the upper and then reassemble. I use a Geissele reaction rod for torquing bbls and a KAC URX vise block to keep the rails in alignment when tightening down the RISII bolts. One in a great while I see a receiver that it's threaded extension isn't square & true and that's where the lapping tool is nice.
    Chief Armorer for Elite Shooting Sports in Manassas VA
    Chief Armorer for Corp Arms (FFL 07-08/SOT 02)

  3. #23
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    Went to the range today. After bore sighting and getting things straight, gun still groups left. Just to make sure.... You move the rear sight to the right if your group is left correct? My group was left... I moved the sight right... And my group stayed the same. The following target is after multiple adjustments from center moving rear sight right.


  4. #24
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    What rear sight, what distance, and how much did you adjust?

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    Jack Leuba
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  5. #25
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    Troy rear. 25 yards so that I could actually see what was going on with the gun. I bore sighted and the initial group was left. I moved in total I believe 18 clicks from the bore sighted position with no change in groups. At this point I'm kind of lost. Unless I'm moving the sight the wrong way.........but 18 clicks total should have had a wider dispersion than what is on that target at that distance I believe.

  6. #26
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    Quote Originally Posted by TylerD View Post
    Troy rear. 25 yards so that I could actually see what was going on with the gun. I bore sighted and the initial group was left. I moved in total I believe 18 clicks from the bore sighted position with no change in groups. At this point I'm kind of lost. Unless I'm moving the sight the wrong way.........but 18 clicks total should have had a wider dispersion than what is on that target at that distance I believe.
    Dude please put an optic on the gun at 25 yards and try again. You should be able to nearly 1 hole it with a 3x

  7. #27
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    Yes, when you shift the aperture right, the strike of your round should shift rightward.

    Presuming your Troy rear is the same as mine: It'll have a movement indicator on the right side, just above the adjuster itself. There should be a letter "R," and an arrow that indicates turning the knob clockwise will shift the strike rightward (because that moves the aperture rightward, as viewed from behind or above the sight). Turn counterclockwise = strike of the round shifts left (aperture moves leftward).

    The conventional wisdom is to tell folks to ignore which direction whatever goes when you turn whichever doodad....and to just follow the arrows to move your groups.

    This is unlikely, but can be easily checked, if for no other reason than to eliminate it as a possibility: Is the rear actually moving when you crank on the adjuster? Can you see it shift in either direction, preferably both directions? If it's NOT moving, would fit the symptoms, and is easy to fix by trying another rear (...or, as suggested, a known-good optic). Many of us have run across dead adjusters on optics, and while I've never seen that occur on an iron rear, it's still possible.
    Contractor scum, AAV

  8. #28
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    Dude please put an optic on the gun at 25 yards and try again. You should be able to nearly 1 hole it with a 3x
    I'm sure the gun can.... And I know with an optic I'll eliminate the issue but this negates having backup irons. Just trying to sort the issue as I haven't had the problem before on my other rifles.

  9. #29
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    Quote Originally Posted by JSantoro View Post
    Yes, when you shift the aperture right, the strike of your round should shift rightward.

    Presuming your Troy rear is the same as mine: It'll have a movement indicator on the right side, just above the adjuster itself. There should be a letter "R," and an arrow that indicates turning the knob clockwise will shift the strike rightward (because that moves the aperture rightward, as viewed from behind or above the sight). Turn counterclockwise = strike of the round shifts left (aperture moves leftward).

    The conventional wisdom is to tell folks to ignore which direction whatever goes when you turn whichever doodad....and to just follow the arrows to move your groups.

    This is unlikely, but can be easily checked, if for no other reason than to eliminate it as a possibility: Is the rear actually moving when you crank on the adjuster? Can you see it shift in either direction, preferably both directions? If it's NOT moving, would fit the symptoms, and is easy to fix by trying another rear (...or, as suggested, a known-good optic). Many of us have run across dead adjusters on optics, and while I've never seen that occur on an iron rear, it's still possible.
    I'll try to get to the range tomorrow and steal a sight off my other rifle and see if this works.

  10. #30
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    As to your zero, choose a zero based on your needs, not the length of the barrel. I just had a shooter with a BCM 11.5 in a class I taught reliably and accurately hit targets at 175 and 330 yards using an Aimpoint. My 10.5 Mk18 has hit targets at 435 yds with open sights. The previous mentioned shooter used a 25 yard zero, while I use a 50/200. Just because its an SBR doesn't mean its limited.

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