Eye Relief Between AR10 and AR15

Discussion in 'AR15/10 Rifles' started by jagsr71, Nov 10, 2011.

  1. jagsr71

    jagsr71 Member

    Messages:
    7
    Joined:
    Nov 9, 2011
    This is my first post here. Great site and some very good reads!
    I have just purchased a DPMS chambered in a .260 Remington. It’s an absolutely beautifully made rifle. I love this caliber and can’t believe it’s not more popular!
    My question is regarding a Sightron SIII SS 6-24X50 LRMOA scope I just bought for it.
    On my other AR15, I mounted a Nikon Monarch scope with a one piece Larue LT-104. It's cantilevered forward for proper eye relief (as that is an inherent issue with AR's).
    However, I was recently informed that the AR10 (style) rifle does NOT have that issue and that the mount need NOT be cantilevered forward.
    Is this correct?
     
    Last edited: Nov 10, 2011

  2. jagsr71

    jagsr71 Member

    Messages:
    7
    Joined:
    Nov 9, 2011
    I'm going to answer my own question here since I've done extensive research in this area.
    It doesn't hurt to get a cantilevered scope mount (like a Larue LT-154 or LT-104) on this style of rifle. It all really depends on how good the eye relief is on your scope. The eye relief is excellent on my new Sightron SIII SS 6-24X50. Therefore it's not really necessary in my case. Also, because this scope also has wonderful windage elevation travel of 100, purchasing a scope base that has a built-in MOA cant for long range shooting is probably not necessary.
    However, if your AR-10 style rifle does need a base that has a certain amount of MOA cant, a few that will work really well are the AADMOUNT (w/ 22 MOA built-in) and the Larue LT-112 (w/ 20 MOA built-in) and the NF Unimount (20 MOA). The LT-154 is canted 10MOA and is also cantilevered.
    Last note here is that a tactical latch may be desirable for scopes that are mounted too far back or that are too low (to comfortably pull the charging handle).
    There many of different brands available.
     
    Last edited: Nov 14, 2011

  3. LouBoyd

    LouBoyd Well-Known Member

    Messages:
    770
    Joined:
    Oct 15, 2007
    I couldn't tell that you had asked a question in your first post. Just statements. In the second though I'll take exception to your use of terms. Cant is a rotation about the axis of the bore. It's not likely you'd get a scope mount to correct cant. As to dimensional differences between a DPMS LR(() and an ar.15, the DPMS receiver is approximately 1" longer than that of AR-15 That is divided up about 1/2" (or a bit less) behind the trigger and about 1/2" (or a bit more) in front of the trigger. it depends on the position of the rigger shoe you use. So if you put the same fixed length stock on both receivers, it will result in about 1/2 more length of pull (shoulder to trigger) than on an AR-15. If you mount mount the scope in the same location on both with respect the front of the receiver, you should get a cheek weld with respect to your trigger hand such that the scope has about 1/2" more eye relief though the scope will be about 1" further from your shoulder. It's not exact because different people hold differently. Both AR-15's and DPMS LR-() recievers have their rails parallel with the rifle bore, and both can accept an inclined (or wedged) rail (not canted). Another difference is that vertical spacing from the AR-15 bore center line to the Picatinny rail top face is a little smaller than on an DPMS LR-(). If that's too high you can also get rings made in a single piece which give the scope inclination (20MOA typical) without the intermediate inclined spacer. Of course, you don't have to use the same length stock on an AR-15 and an LR-() so you can have any length of pull and eye relief you want.

    Not that any of that matters. Just pick the rings and mount them where you get a comfortable cheek weld and your eye properly centered and spaced from the scope's eyepiece. I've never needed a cantilevered rail for either an AR-15 or a DPMS LR-(). Perhaps it's needed for some scopes or for some people's facial shape.
     
    Last edited: Nov 15, 2011
  4. jagsr71

    jagsr71 Member

    Messages:
    7
    Joined:
    Nov 9, 2011
    This was my original question on my first post.



    Yes. You are absolutely correct. What I was referring to was forward cant. Some of the scope bases I mentioned also have a built-in forward cant in order to allow more MOA travel in your scope for longer distance shots. Such as this base below from Badger Ordnance:
    1-Piece Maximized Picatinny-Style 20 MOA Elevated Scope Base Remington 700 Short Action
    Aside from the term forward cant, I have also witnessed other terms such as forward slope or taper.
    In all honesty. I probably shouldn't have used the term "cant" since it's open to interpretation (although I had specifically mentioned MOA cant). What I should have specified is "forward cant."

    Thank you for your response and your detailed description of the different dimensions between the AR-15 and AR-10. Your explanation about the scope having about 1/2" more eye relief and that the vertical spacing from the AR-15 bore center line to the Picatinny rail top face is a little smaller than on an DPMS LR really makes absolutely perfect sense.
     
    Last edited: Nov 15, 2011