Drilling & Tapping Scope Block Holes

By Wayne McLerran

3/2024

New scope block holes are commonly required for a couple of reasons.  You’ve been using iron sights and your aging eyes now require a scope, but your rifle was not setup for mounting a scope.  The rifle was drilled and tapped for a scope but not for the length of scope you have or plan to use.  My situation is similar to the last example.

Browning Black Powder Cartridge Rifles (BPCRs) came from the factory drilled and tapped for a mount spacing of 4” for modern internal adjustable scopes.  Since the minimum spacing for external adjustable scopes is 7.2”, there are two options.  Purchase a scope mounting rail adapter from Steve Earle or have new holes added.  Steve Earle’s solid one-piece rail adapts the 4” hole spacing to 7.2”, eliminating the need for drilling new holes.  The rail works fine for scopes of up to 23” or so and for target distances out to 800yds or a bit more, depending on the type of mounts used.  But for longer distance out to 1000yds, additional rear mount height is required, which is not an option with the rail.

Creedmoor matches are becoming popular and more wide spread around the country.  The Yaupon Creek Silhouette Association (YCSA) near Columbus, TX, of which I’m a member, recently added a Creedmoor match to their monthly calendar.  The distances are 800, 900 and 1000yds.  So, I’ve setup a Browning .45-70 with an 18” 10X J.W. Fecker scope for the longer distances.  New holes were required with a block center-to-center spacing of the 7.2”.  A 3/8” high rear block was used in place of the typical 3/16” high block.  The front block remained the minimum of 3/16” high.  

I should caution anyone considering the same modification.  If the rear block is too high, resulting in too much scope tilt, the image through the scope can be degraded or completely blocked by the shadow of the muzzle and/or front sight.  With my setup, the shadow comes into view and starts to obscure the target image if the rear block is higher than 3/8”.  In that case higher blocks can be used for both the front and rear mounts if necessary.  For more details on setting up a scoped rifle for long-range Creedmoor shooting see my article titled, Considerations & Math for External Adjustable Scopes.

No doubt, over the years many rifles have been drilled and tapped using a common cheap drill press or even a hand drill, some with disastrous results such as drilling completely into the chamber or bore.  If you have a mill-drill or milling machine, especially one with a digital readout (DRO), drilling and tapping holes with precision is within the capabilities of most “shade-tree” gunsmiths, an idiom and reference to the hobby mechanic that can be found working on their vehicle in the driveway under a shade tree.  It’s a group of hobbyist gunsmith that includes myself.  Having completed the modification, I thought readers might be interested in an overview of the process along with some guidelines.

The minimum hole depth rule-of-thumb is one diameter of thread.  I.e., using a 6-48 screw with a major diameter of .136”, the hole should be a minimum of .136” deep which is about 6 ½ threads.  Of course, a deeper hole works as long as there’s sufficient material (safe margin) between the bottom of the hole and the chamber or bore.

Following is a Safe-Margin Chart for the three most common block screws:

screw size------over chamber---------over bore

-------#6 ------------.075”---------------- .050”

-------#8 ------------.100”---------------- .075”

-------#10 ----------.125”---------------- .100”

To determine the maximum safe hole depth, use the following formulas:

Wall Thickness = OD minus ID divided by 2

Max Safe Hole Depth = Wall Thickness minus Safe Margin

An example of a max safe hole depth:

Drilling holes for 6-48 screws over a .45-70 round barrel with an OD of 1” and .459” groove diameter: Max Safe Hole Depth = (1” – .459”)/2 = .270” – .050” = .220”

Once the hole is drilled to depth, start threading with a taper or plug tap, then finish to the bottom of the hole with a bottoming tap.  Of course, if the hole is sufficiently deep a bottoming tap may not be required.

Many shooters don’t have the ability, necessary equipment or desire to attempt this modification, especially with a rifle worth $2,000 to $3,000 or more.  In that case send the rifle to Lee Shaver for a “professional job”.

Wishing you great shooting,

Wayne