By Wayne McLerran
TexasMac's Web Site
Determining the angle of the shoulders is a different matter.  Dovetails
may be cut with various shoulder angles, especially in older antique
firearms.  Although all the dovetails I've measured and worked with had
60 degree shoulders, a cast will help to confirm the correct angle.  One
other measurement that might be needed is the height or depth of the
slot, which is easy to measure with a depth micrometer or the back end
of a caliper.

Wishing you great shooting,
Last update: 9/5/2016

Those of you that have purchased or are considering picking up one of
the recent Winchester Limited Series BPCR manufactured by Miroku
have noted that many come without sights.  The rear receiver tang is
drilled and tapped for a sight and a female dovetail slot has been cut
for installation of your preferred front sight.  Most sight suppliers are
familiar with the Browning BPCRs but may not understand that the
more recent Winchester BPCRs are identical to the Browning’s including
the dovetail slot dimensions.

There are many sizes of dovetails.  To get an idea of the various
dovetail dimensions go to web site and search for
dovetail cutters.  In the following discussion I will limit my comments to
dovetails on rifles, which is something I know a little about.  In general
there are three or four measurements that define the dimensions of a
dovetail slot: the opening and end widths, shoulder angle and depth of
cut.  The width of a female dovetail slot is measured along the long
axis of the barrel at the bottom of the slot, the widest fore and aft
point of the dovetail.  If the slot is tapered, the right opening, with the
muzzle pointing forward, is slightly wider than the left end.  The
shoulders are angled inward and the slot is cut to a specific depth.  If
the slot is not tapered it was likely cut with a standard dovetail
cutter.  The two critical specifications of dovetail cutters are the width
and the shoulder angle.  The depth of cut is determined by the
gunsmith or machinist.  Therefore a 3/8” dovetail slot with 60°
shoulders is 3/8” (0.375”) wide at the base and the angle from the
base to the inside of each shoulder (or side) is 60°, which is by far the
most common dovetail angle used in modern firearms.

If you check with Browning or Winchester, they will likely inform you
that the BPCR dovetail slot is 3/8” with 60 degree shoulders and is cut
to a depth of 0.090”; all of which are accepted as an industry standard
on modern firearms.  But knowledgeable gunsmiths are aware that
some factory barrel
female dovetails slots are slightly oversized on one
end and usually tapered to allow easier insertion of the male sight
dovetail.  In my book on the Browning BPCRs I measured several rifles
and reported the range of dimensions.  Nominally a 3/8” dovetail will
measure 0.375”, but the right opening of the Miroku/Browning
dovetails ranged from 0.380” to 0.384”.  I recently checked the
dovetails on several Miroku/Winchester BPCRs and confirmed
approximately the same range.  I also measured the left ends and found
the width to range from 0.373” to 0.377”, confirming a slight taper.  
Therefore, be sure to follow the industry standard process of removing
an existing sight by drifting it out from left to right and installing it
from right to left.  So how does one accurately measure
the dovetail width?

If an existing front sight is being replaced the first thought may be, why
not just measure the width of the base of the old sight with a
conventional micrometer or caliper?  The problem is that male sight
dovetail bases are slightly rounded on the edges, if for no other reason
than to remove the sharp edge for safe handling, resulting in a
narrower dimension than the female dovetail slot.  A recent example is
a front sight I removed from a Browning BPCR.  I found the right
opening width was 0.385”, but the width of the male dovetail base of
the sight was 0.376”.  The next thought that may come to mind is to
use the rear or inside measuring jaws of a caliper on the female slot.  
Due to the slight offset and edge thickness of the jaws, an accurate
measurement is almost impossible.  So what’s the answer?  I’m no
expert on dovetails, but there are four techniques that I’m aware of.

If you expect to be cutting numerous dovetail slots and installing sights,
one solution is to order a wire gauge dovetail measuring tool from
Brownells.  The current price of one made by XS Sight Systems (part #
006-101-000WB ) is $54.00 plus shipping.  But the vast majority of
shooters reading this article are not gunsmiths, and working with
dovetails is not a common occurrence.

Another method is to make a cast of the female dovetail slot using
either Cerrosafe or epoxy.  Once the cast is removed, measuring the
width of the male dovetail cast is simple.  I have not used this method
but understand it works well as long as the Cerrosafe is removed at the
proper time prior to enlarging and locking itself into the slot.  If an
epoxy cast is made, it should be relatively easy to remove as long as
the dovetail is sufficiently coated with a release agent such as paste
wax.  By the way, never use a steel punch when removing and installing
sights in dovetails.  A brass punch will work but will likely leave a thin
brass smudge on the sight surface.  And always 1st check to determine
if the sight utilizes a setscrew to lock it into place.  I use a short
section from a hard plastic Delrin rod and have yet to run into a sight
that I could not remove or install.

One quick technique that works that works for me is to utilize a slightly
tapered gauge, cut with common scissors out of thin shim stock.  The
gauge width should taper from a dimension narrower than the slot to
slightly wider than the slot.  Sharpen the long edges of the gauge with
a file or sandpaper.  The gauge is pushed in by hand and "wedged" into
the bottom of the female dovetail until it stops.  Mark the gauge at the
start of the dovetail, pull it out and “gently” measure the width at the
mark with calipers or a standard micrometer.  Carefully using this
technique will result in an adequate measurement.  And don’t forget to
measure the opening and ending slot width, which will also provide the
slot taper if any exists.

The most accurate technique is to use a couple of precision steel pins
of identical diameters, which are placed in the dovetail (refer to the
figure below).  The distance between the pins is measured with a
caliper and used in the formulas below.  Just ensure the diameters of
the pins are small enough to contact the side of the dovetails below
the top edge.  A couple of 1/16” (0.0625") drill shanks should work
fine.  Since the vast majority of dovetails have 60° shoulders, I’ve
simplified the formula so you don’t have to figure out the cotangent of
30° (60°/2).