www.ModulusArms.com | AR-308 Jig Instructions
o Using the low (2-speed) or medium (3-speed) gear option if available.
o Best results are obtained with a more powerful, corded drill.
o Cordless drills will work; however, more than one battery and additional cool down times
may be required.
o For 7075 alloy lowers, the alloy is significantly more difficult to drill – a drill press is highly
recommended.
o Marking or center punching the drill spot on the lower can help increase accuracy. Using the
3/8” drill bit as a center punch, square the bit in the x and y direction with a right angle tool,
and lightly tap the end of the drill bit with a hammer to mark the center.
General Guidelines and Recommendations
o Make sure the drill bits are sharp – sharpen if necessary. Dull drill bits will result in
considerably higher effort and excessive temperatures.
o To prevent damage to the template edges, start the drill when the drill bit is through the
drill guide. After drilling, stop the drill bit before moving to the next hole.
o Frequently clear the chips by pulling the drill bit up but not out of the drill guide. The space
between the drill guide and lower is designed to accommodate the chips.
o Clear the chips between the drill guide and lower after each hole to prevent chips from
accumulating and compacting.
o Cutting oil or WD-40 applied with a toothbrush can reduce the drilling effort.
o After each hole, use the drill guide to check the depth to make sure the drill stop did not
slip.
For the milling process, the Modulus Arms jig is compatible with all common methods such as using a
drill press, router, or mini-mill. Using a drill press as a milling tool is NOT recommended and can be
dangerous. A router is common tool that is affordable and commonly used, making them easy to borrow
or rent. A mini-mill is a significantly more expensive option but results a significant reduction in build
time, and an increase in quality for the experienced user.
Drill-Press as a mill (NOT RECOMMENDED)
Router
o A router commonly used for woodworking with a standard ¼” collet.
o The end mill must be a 4” long, ¼” diameter end mill. Four flutes with less than a 1” cutting
depth is recommended for best results. Longer cutting surfaces can damage the jig. Always
check with the first cutting pass that the cutting flutes are below the jig template surface.
o Routers with variable speed give the bests results, although fixed speed routers can be used.
o Beware of inexpensive routers with a lot of flex – they can result in non-parallel walls and
damaged lowers.
o The results in these instructions were achieved with a Bosch PR20EVSK Colt Palm Grip
Router with the speed set at 4.
o End mill cut in a clockwise fashion. Best results are achieved with light clockwise semi-
circles, trying to keep only ¼ of the cutting surface on the metal to be removed.
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