80% Lower

Different opinions on whether you need to coat the inside or not after finishing the lower.

I feel that as long as you are not using or storing the rifle in a harsh environment and you clean it regularly, it should be fine uncoated. I would find some type of spray on finish that goes on thin to finish it if wanted.

Aluminum exposed to oxygen quickly, within seconds, forms a layer of aluminum oxide.

Keep liquid mercury away from it and you're golden.

The key tool for finishing an 80% lower is a router. A drill press can be used for roughly removing some of the metal from the cavity (and, of course, drilling the selector and hammer / trigger pin holes). An inexpensive Harbor Freight drill press is good enough for these operations. (Actually, you could even do these operations with a hand drill, if you are skillful with one.)

There's no need to apply a finish to the raw metal in the cavity.

The place where jigs fall short, in my opinion, is in regulating the depth of the cut. It's easy to cut too deep and end up with a paper-thin web around the trigger opening (assuming you don't break through altogether). Another drawback in some jigs is a less-than-perfect alignment of the selector hole. This has to line up perfectly with the predrilled detent hole, or else your safety will be skewed.

The only experience I have with 80% lowers is with and 80% Arms jig. https://www.80percentarms.com/products/easy-jig-gen-2-multi-platform-ar-15-ar-9-and-308-80-lower-jig/

With this jig I used a laminate router and the tool kit they sell which uses an end mill but to remove material. It worked flawlessly once I figured out how much I could reliably remover per pass. This also solves the problem of removing too much material. I used a raw receiver and costed the entire thing, including fcg pocket, with Duracoat. Probably would have just used regular Krylon or Rustoleum in hind sight.

Most expensive lower you’ll ever buy...

Grab some Birchwood Casey Aluminum Black, hog it out with a router in the jig, coat it after with the Black. Be merry.

K_Dubbya said:

What 80% do you recommend?

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None, if you don’t already own the tools to do the job, and if you’re worried about cost or common sense.

I have done a couple 80% lowers. Not a big ordeal as long as you are patient. Spend extra time in prep phases as they will effect your end product. Use blue painters tape across the top of the side plates onto the lower forging so that scarf doesn’t get wedged down in between plate and lower and shred your finish. You could also use a thin silicone bead there and peel it off when done, I have tried both and they work a whole lot better than not using a seal at that joint.

On the topic of a drill press, anything will work, just be patient and keep extra bits handy. DO NOT USE CHEAP BITS. They get dull, they deflect easily, and they are more likely to break inside of a hole when they deflect. Spend an extra few bucks and start with nice, new, quality bits. Your best bet for a drill press is to get a piece of angle iron and clamp it down on the bench/table and clamp your jig to that. C clamps are nice because they are quick and easy. Take your time though and you will end up with a nice product to be proud of. Rush through it and you will end up with an embarrassing mess that you spray paint to cover up all the ugly.

AlexanderA said:

The key tool for finishing an 80% lower is a router. A drill press can be used for roughly removing some of the metal from the cavity (and, of course, drilling the selector and hammer / trigger pin holes). An inexpensive Harbor Freight drill press is good enough for these operations. (Actually, you could even do these operations with a hand drill, if you are skillful with one.)

There's no need to apply a finish to the raw metal in the cavity.

The place where jigs fall short, in my opinion, is in regulating the depth of the cut. It's easy to cut too deep and end up with a paper-thin web around the trigger opening (assuming you don't break through altogether). Another drawback in some jigs is a less-than-perfect alignment of the selector hole. This has to line up perfectly with the predrilled detent hole, or else your safety will be skewed.

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Worse than skewed, it can become unsafe. I had to adjust the selector hole on my first lower because it was somehow causing the hammer to hang almost instantly after pulling the trigger. It would allow enough hammer travel to not let the trigger reset, and once you wiggled that safety it was firing at some point in which you could not control. Dangerous as hell. I remedied the problem with an epoxied in shim, and backed that up with an ambidextrous selector which further supported the repaired hole. This is the main reason I have done a few 80% lowers. I did a polymer which was easy to cut but easy to mess up too, then an aluminum which was a lot more in depth process, and then another polymer just to be able to claim success. The aluminum was a beautiful burnt orange matched upper/lower and it’s the one that got scratched all to hell when scarf got down in between the lower and the jig.

Of course I have access to a small , manually operated Bridgeport mill with accessories , so I won't deal with that. I will say that this , which is Kalgard Gunkote , apllied to the inside of the machined lower after it is deburred with at least 220 grit and de greased and then warmed in a oven as directed will really smooth up the well prepped fire control parts installed in them and no oxidation will ever form. In fact spray the whole lower with it in a home made clean card board box with a well shaken rattle can with light coats in 65+ degree area nd then bake it. A lovely medium gray self lubeing surface that lasts about as good as any out there and is thin enough to not cause problems. I have been using it for 35 years.
https://www.brownells.com/gunsmith-...s/gun-kote-oven-cure-gun-finish-prod1150.aspx

K_Dubbya said:

The jig I'm using has depth guides so you apparently cannot over-penetrate once you set the bit.

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Easier said than done.

If you want a quality outcome, even an inexpensive 100% lower will be better than an 80% lower that you finish yourself. And, if you consider the cost of the tools, the 80% lower will end up being much more expensive than the 100% lower.

Let's be honest: the main reason for messing with an 80% lower is to have a gun "off the books."

But, it might make sense to get an 80% lower and not complete it. You can thus have a "dummy" gun that's a placeholder in a collection, if and when real ones are outlawed. This would "shelter" your upper and other parts. (I base this on the "near death" experience we had here in Virginia earlier this spring.)

AlexanderA said:

Let's be honest: the main reason for messing with an 80% lower is to have a gun "off the books."

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Let's not assume too much. Most people make 80% kit ARs for the fun of making something, and it's much easier project that making airplanes, cars, or most other things that come unfinished in a box.

K_Dubbya said:

Brand new to 80%'s I've got my lower and jig and I'm looking for a quality drill press.

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One thing nobody has seemed to mention in this thread is that jigs come in 2 varieties: those intended for a drill press and those intended for a router. Read the instructions for your jig and determine which one you got.

The issue is complicated by the fact that it's possible to mix the tools and use the techniques other than what the makers of the jig had in mind. If a drill press jig has a square top, it's actually easy to use a router instead, for example. But don't let it to distract you from what you're supposed to be doing. You can play with tools once you obtained the experience.

Actually, whom am I kidding. Buying wrong tools and learning from mistakes is the whole point of this endeavor. Just remember one thing: never swipe aluminum shaving with your hand, no matter how tempting it feels.

Based on my experience, I say its a "buyer beware" scenario when buying tools from harbor freight that feature either an electrical cord hanging out or a high pressure line plugging in. YMMV.

AlexanderA said:

And, if you consider the cost of the tools, the 80% lower will end up being much more expensive than the 100% lower.

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If you plan on doing multiple lowers, the price difference starts to shift when you factor in transfer fees. Also, many of the power tools used to make the lower can also be used on other (non AR) projects.

Styx said:

If you plan on doing multiple lowers, the price difference starts to shift when you factor in transfer fees.

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At my local gun show, you can get an Anderson 100% stripped lower for $50. The cheapest 80% lower goes for $65. So the unfinished lower costs more even before considering the cost of the tools. Also figure in the learning curve. You may end up ruining one or more lowers before you get the hang of it.

Bottom line: you don't save money by going the 80% route.

AlexanderA said:

At my local gun show, you can get an Anderson 100% stripped lower for $50. The cheapest 80% lower goes for $65. So the unfinished lower costs more even before considering the cost of the tools. Also figure in the learning curve. You may end up ruining one or more lowers before you get the hang of it.

Bottom line: you don't save money by going the 80% route.

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You can buy 80% lowers shipped to your door via multiple online venders for $35-$45 each zero tax, background check fees, or transfer fees. GunBroker has them in a three pack for under $100. You can't make a blanketed statement only based on one specific gun show's pricing... You CAN save money if plan on doing multiple 80% especially if you already have the tools (which aren't that expensive to begin with).

GunPro has Anderson 80%s for $33.99.

Styx said:

You can buy 80% lowers shipped to your door via multiple online vendors for $35-$40 each zero tax, background check fees, or transfer fees.

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If you buy anything online, by mail order, or using a credit card, you leave a paper trail, which defeats the whole purpose of this exercise. The only way you have a gun truly off the books is to buy it face to face, anonymously, with cash.

AlexanderA said:

If you buy anything online, by mail order, or using a credit card, you leave a paper trail, which defeats the whole purpose of this exercise. The only way you have a gun truly off the books is to buy it face to face, anonymously, with cash.

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It is still unserialized, not logged in an government books/computers, and is still untraceable via a serial number. Still no transfer fees or background check waits... For the sake of argument, a completed 80% lower AR dropped at a crime scene can not be traced back to you, so the purpose of 80% lowers is not defeated even if purchased online. There's no way for the government to know when you purchased it. Even if your home is raided, they can't tell if the 80%s that where found were the ones you just ordered or ordered several years prior...

Would it really be a fatal mistake if you break thru the bottom of the receiver? The fire control parts don’t need to be seated against the bottom, only the springs need it to push against. If you used a self contained trigger group couldn’t you just smash through the bottom with your drill/router? I know it gives crud another path to get in, but it also gives it a place to get out.

If I wanted an AR off the books I would just buy it at a flea market, although I understand some don’t people have that option. I have 0% interest in building an 80% AR based on my experience with a Polymer80 Glock build. It was amusing but by the time I sourced all the parts I would have been better off just buying a used Glock off Armslist or at a gun show.

AlexanderA said:

If you buy anything online, by mail order, or using a credit card, you leave a paper trail, which defeats the whole purpose of this exercise...

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...and?

I plan to do a couple of 80% lowers for reasons that have nothing to do with being "off the books" or saving money. It has everything to do with getting a partial fence lower engraved with markings no one offers. It has to do with the satisfaction of doing the machining myself.

AlexanderA said:

If you buy anything online, by mail order, or using a credit card, you leave a paper trail, which defeats the whole purpose of this exercise. The only way you have a gun truly off the books is to buy it face to face, anonymously, with cash.

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Or completely make your own which is about 8 or 10 levels of difficulty higher... and legitimately requires a mill or a furnace capable of melting aluminum so that it can be poured into a mould to make a casting.