Jig Instructions - Southern Buzzard Arms

Jig Instructions
Thank you for buying our lower receiver machining jig. These instructions apply equally to the
AR-15 and to the AR-10 .308 lower receivers. This jig will allow you to finish your 80% lower receiver
using only a drill press and a few basic tools. Please read and understand the instructions entirely
before starting. Once you start drilling, milling or altering your 80% lower, it becomes a firearm
and all local, state and federal laws apply. Please check local laws concerning 80% lowers and
completed lower receivers.
Using power tools is dangerous. Follow all safety instructions from your tool's manufacturer. Always
wear proper safety gear and eye protection when drilling and milling or using power tools.
Specifications:
Hole Sizes:
Trigger Pin Hole 5/32” (.156”)
Hammer Pin Hole 5/32” (.156”)
Selector Pin Hole 3/8’’ (.375”)
Adapter Plates Thickness: .50”
Recommended Drill Bits and End Mills:
1/8” Drill Bit (.125”)
5/32” Drill Bit (.156”)
17/64” Drill Bit (.266”)
3/8” Drill Bit (.375”)
7/16” Drill Bit (.438”)
1/2” Drill Bit (.500”)
1/4" Carbide Endmill (.250”) Minimum 3.5” Long
3/8” Carbide Extended Reach Endmill (.375”) Minimum 3.5” Long
MIL Spec Depths:
Trigger/Fire Control Pocket 1.250” Deep
Rear Shelf Pocket .630” Deep
Other Materials Needed:
Safety Glasses for eye protection
Lubricant : Cutting fluid or WD-40
A Word About the Manufacturing Process:
These instructions are targeted towards the user with only a drill press, but we strongly advise you to
use the highest quality and most rigid machine set-up that you have available to manufacture
your receiver.
The following illustrations show various possibilities of common machine tooling. The upper left
picture below shows a basic drill press used as a router type machine and this is the method
described in these instructions. The middle picture shows the addition of a vise with 2 axis motion to
hold the jig and receiver more securely and you now can use the feed screws to move the part. The
lower right photo shows a standard Bridgeport vertical knee mill with Digital Read Out (DRO) and a
coolant system. This is the recommended method to machine your receiver for the best results. The
higher level of machine you have generally means a higher level of knowledge and abilities and
therefore, there may be modifications to the manufacturing process as described in this guide. Above
all, good luck, have fun and enjoy your new owner-built firearm!
Whats Included:
Your lower receiver manufacturing jig kit includes a left and right side jig plate, #1 a drilling
adapter, #2 trigger/fire control milling adapter plate, #3 shelf and trigger slot milling adapter plate, (4)
1’’x ¼”-20 Allen Bolts and (4) 2’’x ¼”-20 Allen Bolts.
Using Jig Drilling Adapter Plate
The first step includes drilling shallow holes approximately 1/16” deep using the jig
drilling adapter plate with a 1/8” drill bit. Place your lower receiver into the jig using the four supplied
2” bolts. The two 2” allen bolts go through the front pivot pin and rear take-down pin holes of your
lower receiver helping alignment in the jig. Two 2” allen bolts connect the bottom of the jig walls.
Attach the #1 drilling adapter top plate on the jig with the four supplied 1” bolts as shown above.
Loosely screw in all the bolts. Once snug, use an allen wrench to tighten the bolts in a cross
pattern. Do not over tighten and bend the jig corners.
Use an 1/8” drill bit, drill all holes using the #1 drilling adapter plate to a depth of 1/16”. The
drilling adapter plate is .50” thick. The distance of travel on your drill press should be
approximately .56” from the top of the drilling plate. When your finished, your lower should look like
the picture below with the 42 shallow guide holes.
Remove the drilling adapter plate from the jig. Use the color coded picture below for hole depth
reference. Some drill bit coatings, such as TIN, may clog the drill bit quicker. If you
are having problems with clogging, use a non-coated steel bit and remove the drill bit
completely from the hole often to clear the chips.
Use an 1/8” drill bit, drill the three 3 yellow holes to .615” depth from the top of the
receiver. Don’t drill out the three red holes at yet!
Use an 1/8” drill bit, drill the 36 green holes to 1.240” depth from the top of your lower receiver.
Your lower should look like the picture below.
Before drilling the three red holes, you need to loosen and partially slide out the 1/4” jig
bolt that is securing the lower through the rear take-down pin holes. Do not fully
remove the bolt, slide it out enough to allow the 1/8” drill to clear the bolt. If the bolt is
removed completely the lower may shift out of alignment in the jig. Drill the three red holes to
a depth of .620” from the top of the lower and then re-tighten the bolt (some
80% lowers have this milled out already but make sure it has been milled out completely). Make sure
you have enough clearance between the top of the receiver buffer ring and your drill chuck at full
drilling depth.
Use the color-coded picture below as a guide when drilling the next set of holes.
Using a 3/8” drill bit to drill the blue holes to a depth of 1.240” from the top of your receiver. You
might want to drill every other blue hole in a zig-zag pattern to keep the cutting pressure equal
on both sides of the drill bit. After drilling every other hole, go back and drill the skipped
holes. To avoid the jig from moving or the drill bit from walking, secure the jig to the drill
press table in a vise or use clamps when drilling is encouraged. Be sure to drill very slowly
using light pressure to stay centered in the 1/8 pilot holes and prevent the drill bit from walking
into an adjacent hole.
Using a 7/16” drill bit to drill the brown hole to a depth of 1.240” from the top of the lower.
Using a 17/64” drill bit, drill through the two white holes to a depth of 1.400” from the top of
the lower. These holes should go through the bottom of the receiver as they are the trigger holes that
will be used to mill the trigger slot later.
To ensure you are drilling the correct trigger pocket holes, install the rear shelf/trigger slot top plate
before drilling make sure and mark the trigger holes before drilling.
Drilling Selector, Hammer, and Trigger Pin Holes
Before drilling these holes, make sure the jig is flat on the leveled drill press table. The jig
should be on parallel spacers to allow the drill bit to clear the bottom plate. You may use two
of the top plates for parallels, but be sure they are not near the 3/8” selector pin hole. Clamp
securely to prevent the assembly from moving.
Drilling Selector Pin Hole:
Lay the jig on its left side with the 3/8 bushing facing up as shown above. Using a 3/8” drill
bit, align the selector pin hole on the jig wall with the drill bit, clamp and drill through lower
until you are through the left side.
Drilling trigger and hammer pin holes:
With the jig still on it's left side, use a 5/32” drill bit aligned in the trigger pin hole and drill to a
depth of .70” from the face of the jig wall (just through the right receiver wall). Drill the
hammer pin hole in the same manner. IF YOUR DRILL BIT IS LONG ENOUGH DRILL ALL THE
WAY THROUGH AS YOU DID WITH 3/8 HOLE. IF NOT flip over and repeat.
Drilling Rear Shelf 1/2" Holes
Put a 3/8” (.375) rod through the jig selector pin hole into the lower and take out the bolt
holding the lower to the jig in the rear take-down pin hole. A 3/8 drill but shank will work the same to
align the lower to the jig. This step is to prevent the lower from getting pushed down in the jig while
drilling the 1/2" holes in the shelf area. The drill or rod needs to go in about .3/4” from the jig wall to
support the lower. You do not need to push the rod or drill all the way through the lower.
Using a 1/2” drill bit to drill the 2 purple holes in the picture above to a depth of .620” from the top of
the receiver. You need to drill slowly with light pressure so the drill bit stays centered in the 1/8” pilot
holes. Secure the jig to the table to keep it from moving while drilling these
holes.
When you’re done, your lower should look like the picture below. Most of the fire control pocket
material should be removed. Screw the 2” bolt that you previously removed back through the jig takedown pin holes and re-tighten. Remove the rod or drill from the selector hole. Congratulations, you
are now ready to start the milling process.
Milling the Lower Receiver Trigger Pocket
Mount the 3/8” carbide endmill or HSS endmill in the the drill chuck. Ensure the endmill is secured
very tightly in the chuck because vibration during milling may loosen the endmill causing
damage. Adjust the depth of the endmill so at least 1/4” of the endmill's shank is touching the adapter
plate, see below. If you don’t do this you will cut into and ruin your adapter plate.
Adjust your drill press to a slow speed Approx 500 and 700 RPM. Higher speeds can make it more
difficult to hold the jig and could cause the end mill to kick and cause you to lose hold of the jig,
damaging your lower. Mill very slowly in a clockwise direction with a tight grip on the jig. Press firmly
downward on the jig. Make shallow cuts removing only .250” material in each cut
Continually spray cutting fluid and use compressed air (if available) on the endmill to remove
shavings and keep the endmill lubricated as you work. Gradually increase the depth to 1.25”.
After a few passes it will start looking like the picture below.
If the endmill your using isn’t long enough, remove the top adapter plate if your chuck gets to close to
the plate. The area you just milled on the lower can be used as a guide for the endmill as shown
below. Don’t forget to keep a minimum of .25" of the shank in the newly milled area so you don’t cut
into that already milled area.
Keep milling away the material until it is removed down to 1.250” from the top of the receiver. Check
the trigger fit to make sure it will go into the well before moving on to the next step. Watch for burrs on
the trigger near the trigger pin holes and stone them off if necessary to ensure a proper fit.
Milling the Upper Shelf Area
Install the #3 rear shelf/trigger slot adapter top plate to the jig to begin milling the rear shelf area as
shown above. The Mil Spec depth for milling the rear shelf is .630” from the top of the lower receiver.
Be careful when milling the rear shelf pocket you don’t want to mill into the jig bolt. Leave at least 1/4"
of the endmill shank touching the plate to avoid damage to the adapter plate. After you’ve milled the
area in front of the jig bolt, unscrew and slide out the jig bolt and use the same process used to
secure the jig that was used when you drilled the red 1/8” holes by placing a 3/8” rod or drill shank
through the jig into the lower's safety selector hole. Now mill the rear area of the shelf.
Be careful not to mill into the drill shank or rod that is securing the lower in the jig. Be sure to check
the fit against the upper receiver's rear takedown lug before moving on to the next step. When
finished milling the shelf, screw the jig bolt back in and remove the 3/8” drill shank or rod from the
safety selector hole.
Milling out the Trigger Slot
Using the 1/4" carbide end mill install end mill into the drill press. Use caution and lower the end mill
through the trigger slot adapter plate all the way through the 17/64” holes you drilled earlier
through the lower. The distance from the top of the trigger slot adapter plate to the bottom of the
lower is 2.0”. If you have a billet lower be careful not to go too far and hit the trigger guard. Milling in a
slow clockwise direction, mill out the trigger slot. Pay close attention that the endmill shank rides
along the adapter plate wall.
Uninstall the lower from the jig and you now have your completed Lower Receiver ready for
cleaning and assembly.

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