Below you will find a selection of features offered with Modulus Arms Universal Lower Receiver Jigs. Other jigs on the market present fabrication challenges and lack true universal compatibility. At Modulus Arms, we believe we have designed the safest, fastest, and most accurate jigs for finishing AR-15 and AR-308 80 percent lowers. For more information, or to purchase a Modulus Arms Universal Jig, view available Jig products here:

Heavy-Duty AR-15 Universal 80% Lower Jig

AR-15 Universal 80% Lower Jig

AR-308/AR-10 Universal 80% Lower Jig

 Feature # 1: True Universal Compatibility 

Just because a jig's side plates fit your lower doesn’t mean it will work with your lower. Other jigs on the market lack true universal compatibility, resulting in off centered or non-parallel features.

Problem: The width of an AR-15 lower is not a controlled dimension. Forged lowers vary in width from vendor to vendor and from lot to lot. Billet lowers tend to be quite a bit wider than forgings and the width varies from design to design.

Solution: Modulus Arms' jig uses the same lower receiver features that center an upper, resulting in a true universal fit.     


Feature #2: Router Based Milling

Problem: An end mill generates forces during milling that can separate the chuck, causing injury and damage. As a result, the depth of cut recommended with a drill can require 50 shallow passes to cut the main trigger pocket. With the operator’s hand moving the jig, it causes quite a bit of fatigue, and increases the opportunity for mistakes.

Solution: The Modulus Arms jig uses a router for increased control and reduced fatigue that increases the build quality. Furthermore, the router can reduce the number of passes by as much as 8 times to reduce the overall build time.

Feature #3: Fully Supported Trigger Slot for a Properly Shaped Trigger Slot

Problem: Traditional jigs mill out the fire control pocket before the trigger slot, removing all supporting material. As a result, the end mill is a far distance above the thin metal that needs to be cut. When the end mill 1st makes contact with the metal, it tends grab the metal and flex, cutting notches outside of the template area.

Solution: The Modulus Arms jig uses a fully supported process for all milled features. As the trigger slot is cut from top to bottom, the upper material prevents the end mill from flexing outside of the template area.

Feature #4: Reduced Drilling Time

Problem: Quite a bit of material needs to be removed to safely mill the lower. With other jigs, this typically requires up to 42 holes to be drilled twice. On the second go-around, each hole requires that the vice that holds the lower to be adjusted and clamped since the holes overlap. Careful alignment is needed to minimize the drill bit walking.

Solution: The Modulus Arms jig uses 10 strategically placed holes that can be quickly drilled – saving quite a bit of fabrication time.

Feature #5: Depth Gauge

Problem: With a drill press or hand drill, there is no easy way to control and set the depth to the required 1/100 accuracy when the drill is calibrated in 1/8”- 1/16” increments. Depth gauges that can measure depth to 1/100th of an inch are expensive.

Solution: Modulus Arms provides a depth gauge that sets the final dimension for both the drill and end-mill for all three required depth controlled features. Furthermore, the gauge is also used to indicate the top of the lower and the maximum depth of cut.