Scope Block Hole Depth...What Is "safe"?
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  1. #1
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    Default Scope Block Hole Depth...What Is "safe"?

    I have a question for those "in the know" on this subject. When drilling and tapping for scope mount blocks or sights on a center fire rifle barrel, is there a known or accepted safety margin for how deep the holes should be drilled? To set this question up lets say I have a .30 rifle barrel that has a .750 O.D. throughout the entire length of the barrel from receiver to muzzle end. I want to install Unertl style scope blocks and obviously I don't want to drill the mounting screw holes into or too close to the bore. Is there a given safety margin or hole depth (undrilled material between bottom of drilled hole and bore) where a person should stop before getting too close to the bore of the rifle? I hope I haven't confused anyone with this question, but I have seen some tremendous differences in drilled/tapped hole depth on some barrels and I am just wondering if there is an accepted "standard" where one would say "that hole is getting too close to the bore".

    I know in a perfect world holes should be drilled 1.5 times the diameter of the screw to be installed, but sometimes there isn't that much barrel material. So what is too close to the bore for a drilled hole before pressure from a fired round would become a concern? I hope this question makes sense.

    Thanks, Rick

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    Well the rule of thumb I have always gone by is .075 minimum thickness from the bottom of the hole to the inner barrel wall on the front half of the barrel and .100 minimum thickness on the back half of the barrel. One diameter of thread is about all you need for depth on 6x48. You begin to reach a point of diminishing returns beyond that.

    .750-.308=.442 .442/2=.221 wall thickness. So .221-.075=.146 maximum drill depth. A 6x48 screw is .1365 OD so you can just squeeze it by. I would bottom tap the hole. Some manufacturers and gunsmiths will leave a thinner area between the bottom of the hole and the inner barrel wall. I have seen some that are paper thin and they get away with it, but I'm a bit of a scardy cat.

    ADD NOTE: One screw width on 6x48 is about 6 1/2 threads

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    Just did a practice barrel (in school) and the above formula is exactly what we are being taught. The only difference is there are different safe depths for
    different screw sizes for drilling depth above the chamber versus at the muzzle end....can't think of the exact numbers offhand. Perhaps someone
    else will know.

    OD-ID / 2 - safe depth. Different screw sizes have different safe depths dpending on where they are (muzzle vs over chamber). I'll try to get a pic and
    post the info Monday.

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    Quote Originally Posted by rinn69 View Post
    Just did a practice barrel (in school) and the above formula is exactly what we are being taught.

    Quit school and go to work for the Chucker, Problem solved

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    NO MORE APPRENTICES ! I got tired of that a long time ago. Besides, the government won't let us beat them any more.

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    Thank you gentleman, I really appreciate the information to help answer a long standing question in my head. I will be standing by for your followup rinn69.

    Rick

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    HERE IT IS:
    OD minus ID divided by 2 = wall thickness
    Drill depth (MAX) = wall thickness minus safe depth

    SAFE DEPTH:
    screw size--------over chamber--------elsewhere
    -------#6 ------------.075---------------- .050
    -------#8 ------------.100---------------- .075
    -------#10 -----------.125---------------- .100

    Enjoy !!!
    Last edited by rinn69; 02-18-2013 at 07:07 PM. Reason: to coerrect spacing


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