An 80 percent receiver is simply one that is not finished enough to be considered a firearm by the ATF. To the Feds it is a chunk of aluminum and can be purchased online without an FFL. You can finish it for your personal use as long as you may legally own a firearm but you can't sell it unless you're a licensed manufacturer.
Finishing a lower requires a milling machine or drill press, a fixture if you choose the drill press method, some drills and end mills and very basic machine skills. You're not going to save money. The beautiful billet lowers in the pictures came from Juggernaut Tactical. They currently sell the 80 percent lowers for $155 and CNC-finished lowers for $165, so cost isn't a factor.
I think most people tackle a project like this because they like making their own stuff. It's the sense of accomplishment more than anything else. It's going one step further than simply assembling a rifle kit from finished parts.
Over the years, I have built several 1911 pistols for my personal use. I didn't save any money. In fact, a gun assembled from individually purchased parts may cost you more than a factory-built firearm. Still, there is that satisfaction of building your own.
Finishing one of these receivers on my Grizzly Mill/Drill was not a big chore. AR-15 lower receiver blueprints are readily available online, and my main concern was calculating the dimensions from the drawings correctly. The holes for the selector (3/8") and trigger and hammer pins (5/32") were drilled first and they must be precisely positioned for proper function of the fire control group.
Once the holes were done, it was simply a matter of milling out the receiver pocket with a long 7/16" end mill and finally cutting the slot for the trigger with a long 5/16" end mill. Inside dimensions of the pocket are not super critical, and a few thousandths error probably won't be noticed.
One error I made was not cutting the rear channel deep enough into the buffer tube threads, and when I checked the fit of an upper receiver the takedown pin wouldn't quite line up. I milled in a little more clearance and the problem was solved. I would rate finishing one of these receivers on a mill as well within the capabilities of any hobbyist with basic machine skills.
Finishing a lower on a drill press is also possible. You will need a fixture that consists of plates that clamp around the receiver to guide the drills and end mills. The fixture from Juggernaut Tactical is very well made with the side plates drilled for correct positioning of the trigger pin, hammer pin and selector holes.
Three separate top plates are included for different steps of the machining process and a tool kit is available with the fixture which includes the drills and end mills you will need. Most of the aluminum is removed by drilling overlapping holes with progressively larger drills.
It is important to be able to set the depth on your drill press accurately with a dial caliper. In the final steps, the drill press is used as a router as the fixtured receiver is clamped in a vise and moved around the cutter following the pattern of the top plate.
You will need to maintain a solid grip on the vise as you move it around on the drill press table using the slots in the fixture as a template. A good video of this operation is available at the Juggernaut Tactical web site.
Test-fit an upper receiver to the lower when you think it is finished to make sure the takedown pin holes line up and the upper slides into position easily. Also, test fit a standard mil-spec hammer, trigger and selector to check function of the fire control group. The freshly drilled trigger and hammer pin holes will not be anodized against wear, so non-rotating pins may be a good idea for the final build.
Here's the raw 80% receiver as received from Juggernaut Tactical. The first step in the process is drilling the hammer pin, trigger pin and selector holes.
Norcross says finishing an 80% receiver is not a challenging job, but care must be taken to properly position the drilled holes. Check your measurements.
The receiver pocket is milled out with a long 7/16" end mill. Watch your depth! Keep in mind that the freshly cut parts surfaces will have no anodizing.
The finished receiver pocket. Not bad. With a secure setup and proper tool speeds, you can cut a nice, smooth pocket that looks as good as the factory version.
A hammer, trigger and selector are test-fitted to the completed receiver. Everything fits. A properly fitted 80% receiver can look as good as the factory unit.
If you finish your receiver in a drill press a fixture will be required. Note the clearly marked holes on the Juggernaut fixture. You can't screw it up.