MEASURING DOVETAIL SLOT DIMENSIONS
By Wayne McLerran
TexasMac's Web Site
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 Brownells.com 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 factory dovetails are slightly
oversize on Miroku rifles.  They are also tapered.  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
vernier 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 vernier 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 vernier 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 vernier 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).




















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 vernier caliper.


Wishing you great shooting,
Wayne