BROWNING/WINCHESTER BPCR DOVETAILS
& MEASURING DOVETAIL SLOT DIMENSIONS
By Wayne McLerran
TexasMac's Web Site
Last update: 6/5/2011

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
degree shoulders is 3/8” wide at the base and the angle from the base to the
inside of each shoulder (or side) is 60 degrees.

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 when removing an existing sight.  With the muzzle
pointing forward, drift it out from left to right and reinstall 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 most male sight dovetail bases are slightly
rounded on the edges, resulting in a narrower dimension than the female dovetail
slot.  A recent example is a front sight I removed from a Browning BPCR.  Using
my preferred technique, detailed below, to measure the female dovetail, I found
the 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 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
do you do?  I’m no expert on dovetails, but there are three 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 is $50.00 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, when removing and installing sights in dovetails,
never use a steel punch.  A brass punch will work but will likely leave a thin
brass smudge on the sight surface.  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.

The dovetail measuring technique I prefer utilizes 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.  Using this technique
to measure the opening and ending slot widths will also provide the slot taper if
any exists.

Determining the angle of the shoulders is a different matter.  Dovetails are cut
with various shoulder angles.  A cast will help to confirm the correct angle, but
all the dovetails I've measured and worked with have had 60 degree shoulders.  
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