Modulus Arms is new to the 80% lower jig manufacturers. Their Modulus Arms Universal AR-15 80% Lower Receiver Jig is unique in many ways. This is my fourth jig (yeah I know) and I've found that not every lower fits every jig ... until now. Because of the way Modulus Arms jig is designed it appears that they've solved that problem.



Every 80% AR-15 lower that will work with a standard upper and receiver extension has two points where the upper has to index: the front take down tab and the buffer tube/receiver extension tapped hole.



There is a round plug that treads into the buffer tube which allows the jig to index off the rear of the receiver.



And another jig part that connects into the front take down notch where is it held in place either with the take down pin or with a 1/4" aluminum dowel like I used.



Together these two parts index and hold the cutting jig in place for the three areas that need to be cut - the rear shelf cut to 0.620", the trigger hole cut all the way through, and the main pocket for the trigger cut to 1.249" deep. In the photo below the jig is oriented to cut the first two of those pockets - the rear shelf and the trigger holes. Not shown for clarity are the side supports. To cut the main pocket the top of the jig is unscrewed, rotated 180 degrees and re-attached. A very simple and well thought out system.



I had a couple of 80% lowers waiting to be finished so I fired up the mini-mill to have at one. The Modulus Arms jig is designed to be completed with a drill/drill press and either using a handheld router or a mill. The top surface is designed to lay a small router up on top and use a 1/4" end mill to complete. I recommend using a variable speed plunge type router if you're going to use ... this route as you'll want to cut with a lower speed on aluminum than crazy wicked speeds of a fixed router and the plunge adjustment will make using the include depth gauge easier.

Milling out the pockets is a lot of work on the tool which can be done with a cheaper/faster drill bit. Modulus sells a small kit with the required 1/4" carbide end mill, 3/8" drill bit and drill stop, a 19/64" bit for the trigger hole, and a 5/32" for cutting the trigger and disconnector side holes.  Available here.

I connected the jig up and drilled out parts of the pockets using the include depth guide. The 3/8" drill stop is used to limit the plunge depth of the drill press.



3/8" holes are cut in the rear for the shelf that the upper drops down against for the take down pin.



The 19/64" bit cuts the trigger hole all the way down through the bottom of the lower. The milling of the trigger hole is then done. This is different that many other jigs in that the bulk of the pocket material is still there supporting the end mill cutting tool. In some of my lowers when completing the trigger hold with the pocket already cleared the cutting tool wants to cut "outside the lines" as you're using a small 1/4" mill at close to 2" of length.

A side view



The result is pretty nice.



You then mill out the rear shelf. The jig is held up above the surface of the lower which does two important things. First and most importantly it protects the jig from the cutting action of the end mill. Having the tool below the jig's side makes it impossible to nick the jig. Secondly the space allows the swarf a place to gather and pour out so that the work is easier and the upper surface of the lower is protected. In the photo below I've removed the cutting jig to rotate it around for the main pocket work.



And results.



The included drill bit guide mounts under the jig template to guide the 3/8" and 19/64th holes. Starting and stopping the drill press while inside the holes protects the guide from getting cut up too badly. The jig includes all the needed cap screws.



Making lots of chips with the drill saves the wear and tear on the end mill tool.



Milling is done with a 1/4" end mill.



After a bit of milling out pops a completed fire control pocket.





Another unique feature of the jig is that the safety/selector, trigger pin, and disconnector pin holes on the jigs side are provided twice - there's a set on top and rotating the left to right side and flipping them over the bottom pair can be used as well. That's a pretty neat feature.



The results look very nice.





The Modulus Arms web site indicates that they're working on a .308 upper.

This jig solves the issue of having to match your jig to your 80% lower. It can be done simple tools like a drill press and a handheld variable speed plunge router or a mini mill. The fit and finish of the jig is top notch and shows that the designer has thought out the 80% completion process completely. I can most highly recommend this jig.

Yikes. I'd have to look at my jigs but I think only my 1998 era Tannery had that feature as those "80%" lowers were closer to what we call 40% ... of course back then we didn't have electric motors and the lowers were made out of stone ...

I'll have to pull out my other jigs to see if they index the bolt catch plunger.

I've made a couple of lowers using a drill press ... but that was before the jig science changed allowing us the ability to use a router. The drill press is designed to do vertical up-n-down work. The bit is designed to be pushed down into the work. With a router (and of course the mill) the work is designed to be pressed sideways into the tool - this is what we're doing for the majority of the work on the fire control pocket. Because of the difference the router/mill has less slop side-to-side than an equal drill press.

If I didn't have my mill I'd rent a variable speed plunge router at the local Home Depot for the day. You could likely finish several lowers in that time.

They're like potato chips ... once you do one ... you do another and another ... serial numbers are so overrated.

I've seen lowers like that ... weren't those designed to prevent the use of a lighting link or something?

I've chased 'trons all my life ... not turned wrenches. I am not a machinist by any measure.

You're about 99% likely to complete the lower good enough that it will work ... about 97% to do it flawlessly. I've now made more than a handful of 80% lowers and only had issues with my first one back in 1998 when those lowers required drilling out all holes - including finishing the magazine well (uggh!) and the buffer extension hole. Using today's 80% lowers and a jig like this one you're going to have to work hard to not get it finished correctly. The best feature of this jig is that it's design is "outside-the-box" in that it indexes (and holds) the 80% differently. The thickness of the side walls (the width measurement of the lower) varies a bit as does the design of the magwell flare and trigger guard (some have them others don't). The Modulus Arms grips the points where the industry standard upper meets the lower. You then attach the side walls - there's a bit of movement allowed until locked down to allow for different width lowers - index the rear take down pin (EXCEPT WHILE MILLING THE REAR POCKET!!) and tighten up.

The nice thing about doing the work at your friends shop is that they're good tools there to use. There are wonderful videos on the Modulus Arms site and following them step-by-step makes it easy to complete. A good machine shop is going to have the vice to clamp the work solid, cutting fluid, and a good mill. I cut aluminum at 1250 RPM and use a cutting fluid to keep things cool.

You're set.

My mill has the extended height modification but still I'd rather clamp the jig directly to the table. A mill vice big enough to hold the jig is going to be a big (expensive) one. The clamp kit is cheap and locks tightly - the only negative is ensuring that you've got the lower aligned correctly before locking it down and I use a series of mill squares to square the jig before tightening everything down.