Install:  Arca rail on Bergara HMR style stocks


Bergara HMR stocks have an aluminum structure molded into them, entailing a bar along the centerline of the forend to allow for greater support of the arca rail screws.  However, the bar may present a suprise obstacle for arca rail installation.  The arca rail utilizes the sling stud screw locations for the front two attachment points but requires adding two more screws to support the rear end of the arca rail.

There are two methods that can be used with the included hardware to fasten the arca rail to the stock.  The aluminum bar in the stock can be drilled and tapped for the 10-32 mounting screws, or the forend can be drilled with an oversize through hole which allows T-nuts to be inserted in the barrel channel to work in conjuction with the mounting screws. 

Drilling and tapping threads into the bar is a clean install, but requires a higher degree of precision on placement and alignment of the threaded holes.  The drill and tap option is perhaps best left to those with a milling machine or ridgid drill press and vise setup.  Using the t-nuts allows some tolerance on the hole locations, as the t-nuts can float in the through holes and self center with the installation of the arca rail.

The following installation is performed utilizing methods and tools that are most achievable by end users without a machine shop at their disposal.  While a milling machine is shown here for drilling the holes, the same methods could be used using a drill press and vise.  A hand held drill could even be used with great care to facilitate hole placement. 

Note that there are two variants of the arca rail designed for the short action HMR style stocks.  The screw locations are the same for both sizes of arca rails.  The arca rail variants are basically a light "hunter" rail, matched to the forend length, and a heavier "match grade" rail that extends forward of the forend for more bipod length and rail real estate.

If you landed on this page looking for an arca rail, snag one at the link below.


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Download technical drawing.

Bergara HMR Install Technical Reference Sheet

Layout screw locations.

Place some masking tape on the forend where the rear screw holes will need to be added.  Then, temporarily attach the arca rail using the front two screws.  Take care to maximally center the front screws in the arca rail slots.  The swallow tail cut at the back of the rail should be centered over the bottom metal geometry.  Make alignment adjustments as needed and tighten the front screws to hold the rail in place.

With the rail in place, the slot locations for the rear screws can then be transfered to the masking tape which will show the hole locations once the rail is removed, as shown below.  You can confirm the locations by measuring the distance from the sling stud screws and referencing the the drawing.  If you do have more precise tools and ability, take it from here!

New hole locations after using the arca rail to mark the slot locations.  Center the new holes in the slots.
Confirming locations for the new holes.

Level and secure the stock for drilling.

For best results, the axis of the holes should be perpendicular to the arca rail and bottom plane of the forend.  With the stock upside down, level and hold the stock in a vise on a drill press table or milling machine.  Using an angle finder or bubble level, level the forend bottom in both directions when clamping the stock in a vise, as shown below.
Leveling the forend while clamping the stock in the vise.
Checking level on the second axis.

Drilling the holes.

Start the holes with a center drill, drillling deep enough to hit the metallic bar inside the stock.  If possible, change drill bits and complete the operations below at the first location before moving to the second hole. 

For the drill and tap method, drill the minor diameter through the stock  using a #21 (0.159 in diameter) drill for the 10-32 thread size.  While the stock is held in place for each hole, use the drill press to keep the tap aligned.  Use a tap guide if available. 

For the t-nut method, drill through holes with a diameter around .265" for clearance of the t-nut barrel.  You can always drill these a little bigger if your alignment is off and the t-nuts need more room to float.

The aluminum inside the stock is a gummy alloy.  We recommend using a good cutting lubricant for both drilling and tapping.  In a pinch, WD-40 is better than nothing.
Centering the center drill in the slot location.  The ol' eyeball method...   measuring and marking for precise locations would obviously be better.
Threading the hole, using a pointed tool in the spindle to keep the tap and tap handle aligned vertically.

T-nut  method.

When using the T-nuts instead of drill and tapping, just drill complely through the forend with the clearance drill size.

Friction between the nut and stock material seems to be sufficient when tightening screws, but if needed you can use the tips of the jaws on a small, adjustable wrench to hold the flats on the t-nuts while tightening the screws. 

T-nuts placed in new holes in the barrel channel.
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