• Scope Mounts: Getting It Right The First Time

    One of the most frequently asked questions we see on the forum is in regards to scope rings and bases, more specifically what combination of the two will permit clearance of a scope with a specific size objective lens. Obviously with the limitless number of possible base/ring combinations available this can be a difficult question to answer, but the information provided below should allow you to determine exactly how much clearance is necessary for the scope you have selected to mount on your rifle.

    Background Information:
    In determining what base/ring combination to use, one must first understand a few simple principals. First and foremost, it is recommended to have at least 3/16 inch of clearance between the objective bell of the scope and the barrel itself. While some may see this as excessive, one must remember that room will be needed to install the dust covers or a flip-up style cap to protect the lenses. A 1/8th inch gap usually won’t give you the needed clearance for this, so I always shoot for the 3/16 inch measurement as the bare minimum.

    Second, one needs to understand that each and every base and ring manufacturer builds their rings and bases to different height specifications. In short, Brand A’s medium height ring will not be the same as the height of Brand B’s medium height ring 99% of the time. THIS IS IMPORTANT TO KNOW as nine times out of ten this is where people get into trouble.

    The last piece of the puzzle is the scope itself. Before you can even start looking at bases and rings, you should already have your scope picked out and know its exterior dimensions. This is a key factor in calculating the required height needed of the base/ring combination to provide proper scope to barrel clearance.

    Required Measurements:
    Below is a list of the specific measurements you will need to have to properly determine which base/ring combination will provide you with the proper amount of scope to barrel clearance. I can’t stress enough how important it is that these figures are accurate, as 10-20 thousandths can make all the difference.

    - Scope Tube Outside Diameter
    - Scope Objective Bell Outside Diameter
    - Scope base height
    - Ring height*

    *Special care needs to be taken when collecting the scope ring heights, as there are two different measurements that can be used. The first measurement is the height of the ring from the mounting base of the ring to the bottom edge of the mounting surface for the scope. The second measurement available is from the mounting base to the centerline of the ring. It is imperative that you know which measurement you are dealing with, and that you use the same measurement for all the rings you are comparing.

    Putting It All Together:
    With the explanations out of the way, let’s run through a sample calculation so you can see exactly how all these measurements are used to determine the approximate mounting height. To keep things simple, I will use the generic measurements outlined in the following table.

    Scope Tube Diameter 1.000"
    Scope Objective Outside Diameter 2.500"
    Scope Base A Height
    0.250" Scope Ring A Height* 0.285"
    Scope Base B Height
    Scope Ring B Height* 0.485"
    Scope Base C Height 0.475" Scope Ring C Height* 0.685"
    *Scope ring heights measured from the mounting base of the ring to the bottom edge of the mounting surface of the scope.

    NOTE: I highly recommend you take all your own measurements on the scope yourself rather than relying on published measurements by the manufacturer. This will ensure the measurements are 100% accurate.

    The first calculation we need to make is to determine how much of the objective bell protrudes below the bottom line of the main scope tube. To do this we use the following equation:

    (Scope Objective Diameter – Scope Tube Diameter) / 2
    (2.500” – 1.000”) / 2
    1.500”/2 = 0.750”
    0.750” = Objective Allowance

    Subtracting the scopes outside tube diameter (1.000”) from the scopes outside objective bell diameter (2.500”) will tells us exactly how much larger the objective bell is than the tube itself. Dividing this figure by two (2) consequently tells us how much larger the scopes objective bell is on one side (bottom for our needs) than the scopes main tube. As shown in the equation above, the scopes objective bell in this example extends precisely Ύ inches beyond the outside diameter of the main tube on any side.

    This figure tells us how tall our combined base and ring height needs to be to mount our scope without it touching the barrel. Now I can hear the gears turning in your heads…you’re thinking “won’t using that figure put the objective bell of the scope resting right on top of the barrel with zero gap? The answer is not usually, and here’s why.

    Remember that the barrel diameter at the shank (the largest diameter of the barrel) is smaller than that of the receivers as the barrel nut is the same diameter as the receiver. Also, remember that your barrel has a taper and gets smaller in diameter the farther you get from the action. Since the taper of the barrel is minimal in the area where the scope will be overhead, it is not a major consideration in our calculations.

    The difference in the barrel nuts (and subsequently the actions) diameter to the barrel shank is relative. Specifically, the barrel nut and action are 1.310” in diameter, compared to the barrel shank’s 1.0625” diameter. Using the same formula we did above, one determines that the difference of these two diameters will result in a clearance of right at 0.1235”, or just a hair under 1/8”.

    (Barrel Nut Diameter – Barrel Shank Diameter) / 2
    (1.310” – 1.0625”) / 2
    0.2475” / 2 = 0.1235”
    0.1235” = Barrel Nut Allowance

    However, ones overall scope length will be the deciding factor. If the scope’s overall length does not place the objective bell beyond the location of the barrel nut, you obviously won't have that clearance available. (more information on this is below)

    At this point we need to remember that we are seeking a clearance of 3/16 to 5/16” between the barrel and the scope. To determine the base/ring Minimum Combined Height, we must add 0.1875” (3/16”) to the Objective Allowance figure. Subsequently, to obtain the Maximum Combined Height we simply add 0.3125” (5/16”) to the Objective Allowance figure.

    Objective Allowance + 3/16” = Minimum Combined Height
    0.750” + 0.1875” = 0.9375”
    0.9375” = Minimum Combined Height (3/16” clearance)

    Objective Allowance + 5/16” = Maximum Combined Height
    0.750” + 0.3125” = 1.0625”
    1.0625” = Maximum Combined Height (5/16” clearance)

    Note that I did not figure in the Barrel Nut Allowance for the Minimum Combined Height. Most scopes on the market today have a short overall length, and thus will put the objective bell over the nut. However, if you happen to have a rather long scope and know that the objective bell will not be directly over the nut, you can include the Barrel Nut Allowance mentioned previously in your calculations. In this instance, you would want to modify the above formulas as follows.

    Objective Allowance + (3/16 – Barrel Nut Allowance) = Minimum Combined Height
    0.750 + (0.1875 – 0.1235) = Minimum Combined Height
    0.750 + 0.0640 = 0.814”
    0.814” = Minimum Combined Height (3/16” clearance)

    Objective Allowance + (5/16 – Barrel Nut Allowance) = Maximum Combined Height
    0.750 + (0.3125 – 0.1235) = Maximum Combined Height
    0.750 + 0.189 = 0.939”
    0.939” = Maximum Combined Height (5/16” clearance)

    Now all that is left for you to do is look at the individual heights of the available bases and rings to determine which combination will yield a combined height which falls within our determined range of 0.9375” and 1.0625” from our first example.

    Looking at the base and ring heights specified in the table earlier, we can determine the following examples…

    1. Scope Base C (0.475”) with Scope Rings A (0.285”) = 0.760”
    Obviously, not tall enough for our needs

    2. Scope Base C (0.475) with Scope Rings B (0.385”) = 0.860”
    Still not tall enough, but getting closer!

    3. Scope Base C (0.475”) with Scope Rings C (0.685”) = 1.160”
    Too high!

    4. Scope Base A (0.250”) with Scope Rings C (0.685”) = 0.935”
    Enough Clearance for the scope itself, but may not provide enough clearance for duct covers or flip-up caps.

    5. Scope Base B (0.350”) with Scope Rings C (0.685”) = 1.035”
    BINGO! With roughly just over Ό” of clearance, this setup should provide enough clearance for both the scope and caps.

    This base/ring combination in example #4 above is just under our Minimum Combined Height and will provide only 0.180” clearance. However, as I have stated throughout this article it is best to shoot for a 3/16 to 5/16 inch clearance if you intend to use caps on your scope. Example #5 above will provide adequate clearance for the scope, and also permit the use of most scope caps.

    A few Additional Notes:
    1) This formula is not fool-proof, as in some cases (such as scopes with short tube lengths or when using a tall base with short rings) you could run into scope/base interference.
    2) This formula will require modification to use on any rifle other than a Savage 110 as there will be no barrel nut clearance available.
    3) When using the mounting base to centerline of the ring height measurement, remember to subtract ½ the scope tube diameter from the measurement to determine the actual mounting height of the ring to the bottom edge of the scope tube.
    4) If you intend to use scope caps or flip-up lens covers, I highly recommend installing them on the scope prior to taking your Outside Objective Diameter measurement. This will give you an exact measurement of the objective with the cap/flip-up in place, and you will only need to add in 1/8" to 3/16" for barrel clearance to assure the lowest possible mounting of the scope.

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