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Thread: Compatibility of a rifle mounted RMR and a X300U positioned at 12:00?

  1. #1
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    Compatibility of a rifle mounted RMR and a X300U positioned at 12:00?

    I'm finally going to the effort and expense of setting up an AR for home defense. I have a Trijicon RMR that has been just sitting around since I sold my milled G19 a few years ago, so I bought a Scalarworks 1.42" mount and put the RMR on a 16" AR with a 13.5" rail. The storm kept me from going to the range today, but just handling the rifle around the house, I think the RMR is going to be close to perfect for this role. My next decision is what kind of light get. I'm trying to decide between a Surefire Mini Scout Light Pro mounted at ~10:00-11:00 and a X300U-B mounted at 12:00 in front of the folding front sight. I think I would like the X300U the most because it just seems simpler and tidier than having a light hanging off the side of the gun. However, I worry a little bit about the light intruding into the sight picture of the RMR. If I were running a SRO or MRO or had bought a 1.57" mount, I wouldn't worry about it so much because the dot would be a bit higher. If anyone has run a RMR (or any dot with a similar height over bore) with a light at 12:00, I'd certainly appreciate knowing what you thought of it. Hopefully I'm just overthinking this.

  2. #2
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    While not the same, maybe this can give you a perspective on the height of a TLR1-HL and the irons.

  3. #3
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    Standard “co-witness” optics mounts on AR’s are normally approximately 1.50”.

    1.42 seems low to me regardless of the X300.

    I would definitely try to exchange or sell the 1.42 for a higher mount.

  4. #4
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    Quote Originally Posted by TWR View Post

    While not the same, maybe this can give you a perspective on the height of a TLR1-HL and the irons.
    The Mossie mount height is not the same as an x300 on a continuous rail.

  5. #5
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    @TWR, does your light block much or the view through the dot?

    Quote Originally Posted by HCM View Post
    Standard “co-witness” optics mounts on AR’s are normally approximately 1.50”.

    1.42 seems low to me regardless of the X300.

    I would definitely try to exchange or sell the 1.42 for a higher mount.
    I think it's too late to exchange, but I'll keep the possibility of selling in mind. After I read your comment, I thought to check Trijicon's website. Their description of their full cowitness red dot mount states that the distance from the top of the rail to the optical axis is 1.384". Lower one-third cowitness is 1.511" for the RMR. My calipers are less than ideal for measuring optical axis, but the setup I have looks very to 1.4", which, of course, is less than the 1.5" you mentioned. These numbers would explain why my iron sights block so much of the window when I have them up. I haven't sighted in the RMR yet, so I'm hoping that situation will improve when the dot is in the right place.

    I'm leaning heavily toward the rifle light now instead of the X300. That would make the height issues a non-pressing matter, and I'm surprisingly approaching gun decision fatigue.

  6. #6
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    Quote Originally Posted by pangloss View Post
    @TWR, does your light block much or the view through the dot?



    I think it's too late to exchange, but I'll keep the possibility of selling in mind. After I read your comment, I thought to check Trijicon's website. Their description of their full cowitness red dot mount states that the distance from the top of the rail to the optical axis is 1.384". Lower one-third cowitness is 1.511" for the RMR. My calipers are less than ideal for measuring optical axis, but the setup I have looks very to 1.4", which, of course, is less than the 1.5" you mentioned. These numbers would explain why my iron sights block so much of the window when I have them up. I haven't sighted in the RMR yet, so I'm hoping that situation will improve when the dot is in the right place.

    I'm leaning heavily toward the rifle light now instead of the X300. That would make the height issues a non-pressing matter, and I'm surprisingly approaching gun decision fatigue.
    Forget the light. Go back to your intended purpose, home defense.

    For close range use with a reflex sight such as a red dot, default to higher mounts. Cheekweld and parallax are not critical factors and higher mount gives you a more upright head position which translates into seeing more/better.

    Seeing better leads to better decision making.

  7. #7
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    I understand the mossie mount and TLR are not the same, the same light on my KMR rail sits below the front sight too. As I stated it’s just for reference, if he can see the RMR over his sights, he should be good with the light.

    I don’t have a pic with a TLR on a full rail. Do you?

  8. #8
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    The light does not block much view at all. The PRO is mounted lower 1/3rd so it’s not seen at all.

  9. #9
    Not an RMR, or an X300, but similar setup with Insight WML and Vortex Crossfire in a lower 1/3 mount:

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    (note this is with fixed front and rear BUIS. The front sight, and little bit of the light and my thumb visible behind it, are even less obvious if you are in line with the optic, instead of looking down over the rear sight like the second picture.)
    Anti-astroturfing disclaimer: I am the owner of Bagman Tactical (custom tactical nylon).

  10. #10
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    I don't have an RMR, but I do have a 12 o'clock mounted X300 and an Aimpoint PRO on a 10.5".

    The QRP2 mount included with the Aimpoint is kinda in-between absolute and lower 1/3. I don't have issues with the light blocking FOV, but YMMV.

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