I made my first three 80% lowers back in 1999 with a drill press and hand tools. Since then I've completed both aluminum and polymer lowers. I wanted to try something different and get into the .308 AR so I purchased a DMPS pattern 80% lower from 80 Percent Arms along with a matching jig.
This is my third jig and by far my favorite. It’s designed to allow someone with nothing more than a $100 portable router to finish this upper. I used a mini-mill so that meant I didn’t have to use the upper aluminum plate which acts as platform to rest the router on as it moves round inside the various pockets that need to be milled.
The feature I like the most of this jig is that there’s a spacer plate that is slightly recessed from the template plate that sits on top. This allows the mill’s cutting piece to engage the upper without the chance of cutting into the jig. Another feature I like is that the jig is tall enough to allow the jig to rest flat on the mill’s top surface. Another jig I have allows the trigger assembly to hang below meaning I have to support the jig above the table with 1-2-3 blocks.
The only thing I dislike about this jig are that the horizontal screws are smaller than the ¼” screws that I'm used to. I use the provided screws are strong enough to hold the lower tight, I just like the beefier ones in my other jig better.
The lower was finished to standard 80% completion requiring the trigger pocket to be milled and six holes to be drilled on the sides – the four trigger pins and the two safety holes and to have the trigger hole cut through the bottom of the trigger control pocket. The quality of the lower was good with no rough edges nor blemishes in the finish. A DMPS 20 round magazine dropped from the mag well without issue.
The jig parts. Left top and bottom are the left and right side plates. Center top to bottom - the 3/8" drill bit template, trigger and rear shelf template, the main front trigger pocket template, and the spacer. Right - the top plate for use with a hand router.
The jig parts stacked in order. Bottom to top - the left and right sides, the template spacer, one of the trigger pocket spacers, and on top the router platform.
One of the two built-in depth gauges.
I put the blue tape on the 80% lower to protect it from scuff marks from the mill chips.
The mill has been modified with digital read out’s (DRO) on all three axis but for this project only the Y depth gauge is needed.
3/8" Hole Template's results. When using a hand router to mill out the trigger pocket the drill bit template allows for removal of about 50% of the aluminum using a hand drill. Even with the mini-mill I completed this step as using a drill bit removes lots of material meaning less chip making using the end mill. There is a groove in the front and rear of the template spacer leaving a place for the chips to pour out. The template has been removed to show the holes cut by the bit into the lower receiver for the front main portion of the trigger control pocket.
Making Chips. I use a chip brush and shop vac to pull out the chips. I use cutting oil to prolong the life of the end mill making 0.2" passes clockwise around the templates with a 3/8" end mill. As I get close to the end depth I come within 0.025" and then make a last finishing pass. You do NOT want to cut too deep!
Another feature unique to the 80 Percent Arms jig are the built in depth gauges in the opposite pocket template. This allows someone to very accurately set the depth without expensive gauges or a DRO. The trigger pocket has two levels that it needs to be milled to – 1.249” and 0.630” so there’s two templates. With the template spacer recessed from the size of the pocket template it’s easy to run the end mill around in the template without fear of damaging the jig.
Front Pocket Done. Here’s the spacer and template removed showing the front pocket cut down to 1.249”.
Rear Pocket Template. The rear pocket is milled to only 0.630” and a separate pocket template is provided.
Trigger Template. The opposite of the rear pocket template serves as the trigger hole template. This template is held in place with just two shorter screws without the need for the spacer as the cutting surface is well below the jig’s surface.
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Front and rear pockets done. The round hole in the bottom is the pistol grip screw hole.
Trigger and safety holes use the jig mounted sideways. A quick use of a couple of drill bits and you’re done. I use an edge debur tool to run around the top of the trigger pocket and around the outside of each fire control hole.
I’m still waiting on the lower competition kit and I’m busy shopping for an upper and furniture. Using this jig with the mill made finishing the .308 lower easy. Looking at the intended use with a hand router someone should be able to use a very inexpensive hand tool to do the same work normally performed with an expensive mini-mill. I would advise someone using a hand router to buy a quality cutting bit and make small passes as shown in the instructions using the depth gauges included on the back of the pocket templates.
Total tool time was under an hour.
http://www.80percentarms.com/
The billet 80% lower receivers are $129.95 in 6061-T6 anodized black and are currently on backorder.
The 80% .307 Easy Jig (tm) are $159.99 and are also on backorder at this time.