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Thread: Mounting things more to the rear or front of the rail slot?

  1. #1
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    Mounting things more to the rear or front of the rail slot?

    This past weekend I let someone shoot my M4 as a way of introducing that person to the weapon.

    The person wanted to shoot the rifle with the carry handle sight, so I reluctantly un-assed the Aimpoint T1 and LMT BUIS (I plan to put the Aimpoint on a QD mount so it was coming off in a few days anyway).

    I mounted the carry handle all the way forward on the rail (flush with the front of the upper) and tightened the thumb nuts down as far as I could by hand, and then with a partial turn using a quarter. I know everything was tighter than Dick's hatband when I finished.

    At the end of the range session, I noticed that the carry handle had backed off just a tad. Clearly, it was still mounted and still as tight as I made it, but the carry handle had obviously shifted rearward maybe 1/32 or 1/64 of an inch, just enough to expose a tiny bit of the front edge of the upper receiver. In other words, I mounted the carry handle flush with the front of the upper, and it backed off just a hair.

    I could swear I remember reading recommendations that a mount be situated as far forward as possible because the final recoil impulse has a forward momentum and this should keep the mount from shifting. Clearly this did not happen.

    I also have performed a couple of searches on this forum and I'm either mistaken about what I read or my search criteria just didn't pull it up.

    It's kind of a moot point, because I'll be going back to the Aimpoint anyway when I get the QD mount, but now my curiosity is up. And I figure the same physics are happening to the Aimpoint's mount.

    Should I loosen and re-tighten the carry handle mounted all the way forward and shoot it again to allow things to "seat" better?

    Or has it shifted to its natural position (still good and tight), and I should just leave it alone?
    Last edited by Doc Safari; 09-18-12 at 17:59.

  2. #2
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    Strange. Is it possible your handle got bumped? Or maybe it was the return stroke of your bolt throwing it in the opposite direction? H2 or H3 buffer?

    Just an amateurs speculation.

  3. #3
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    IIRC - Pat MacNamera broached this subject in the class that I attended, and his recommendations were (as I said, IIRC) to pull the attachment to the rear of the rail slots before torquing down. He did explain the why, but I have since lost that info.

    Having said that, I have not noticed any movement on my Aimpoint PRO, and I did no more than fitting it on the rail (neither full forward or full rearward in the slots) and tightened the knob the recommended 3 clicks.
    Sticks

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  4. #4
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    My bad memory also remembers F2S saying to tighten it all the way forward. Hopefully he will come by and correct me if I'm wrong.

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  5. #5
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    I used to notice the same thing with carry handle sights. It may happen with others but it is more obvious with the carry handles i guess. The only thing I could come up with was that it was happening when dropping the bolt on a empty chamber. I suppose that would give the gun a sharp jolt to the front and all accessories due to weight and inertia or some crap like that would travel in the opposite direction. The opposite would be true when shooting the gun due to the forces from the muzzle proppeling the weapon rearward. I can only guess that the force of the bolt going home is slowed by picking up the round from the mag and chambering it. Perhaps this is enough to not cause shifting that you see when dropping the bolt on an empty chamber. Again, this is me guessing what may be going on.

    Question is, was the bolt "dropped" on an empty chamber when you were done shooting before noticing the sight?
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  6. #6
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    I've heard this for a years now. Kyle Defoor talks about pushing it forward as well. I've been doing this with all my optics and irons and have never had one shift.

    Now, the Aimpoint 3x twist mount base, no matter how tight I get it, shifts eventually.
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  7. #7
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    Move it back forward. This just happens sometimes. I run a lot of carry handles, and every so often I'll get a shifter like this.
    "You people have too much time on your hands." - scottryan

  8. #8
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    So if it shifts to the rear and then stays there what difference does it make?

  9. #9
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    The normal way I have been taught to mount anything to a recoiling firearm is to ensure that it is seated forward.

    That being said, drift can and does happen. Setting aside a looseness issue, if the mount is tight, where it stops moving is the right place for it, though in my experience a properly tightened mount that is mounted forward into the lug won't move. The problem with this is that "properly tightened" can be a hard one to nail down, and (much like barrel torque) the user should be mounting optics with a torque device unless he has reached the level of experience that allows him to go by feel. To further annoy the user, more than a few mount manufacturers tell the user to mount with insufficient tension. The carry handle knob type tensioners blow. I have seen numerous (as in, literally, too many to count) "professional" users mount them with the recommended level of torque, only to have them shoot loose, or shoot off. Further, they need to be loctited, not just dry-mounted. This applies to the Trijicon TA51 mount as well.

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  10. #10
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    Quote Originally Posted by Sticks View Post
    IIRC - Pat MacNamera broached this subject in the class that I attended, and his recommendations were (as I said, IIRC) to pull the attachment to the rear of the rail slots before torquing down. He did explain the why, but I have since lost that info.
    I am very interested to hear Pat's reasoning for this.

    Typos brought to you via Tapatalk and autocorrect.
    Jack Leuba
    Director, Military and Government Sales
    Knight's Armament Company
    jleuba@knightarmco.com

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