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5 Biggest Mistakes When Finishing Your 80 Lower

We all make mistakes, it just happens, and we aren’t perfect. Mistakes, when assembling an AR-15 build kit and finishing an 80% lower receiver, can be a real pain. Luckily, it’s harder to ruin an 80 lower than you think. Mistakes will always be made, but with a little know how it’s possible to minimize and reduce the risk greatly.

Wrecking the Finish

This is a very easy mistake to make as you build your AR-15. From the time you start milling an 80% lower to the time you connect upper to lower you are at risk. Marring the finish isn’t a big deal in terms of weapon operability. It’s not dangerous, doesn’t decrease the weapon’s ability to perform, but it looks absolutely terrible. Exposed aluminum can rust and oxidate as well. This is number one on the list because it is the most common issue when finishing an 80% lower.

To avoid this? The proper tools are the most important way to avoid this, more on that later. Use a healthy dose of blue painter’s tape when assembling the magazine catch and assembly. When using your < a href=”” title=”80 lower jig” target=”_blank”>80 lower jig, make sure everything is perfectly in place! A small slip is a big oops.

To fix that mare you’ll need a little refinish. For Cerakote or duracoat this is easy. Anodizing needs to use something like Birchwood Casey Aluma Black. Apply either immediately, or brush scratches light with steel wool and then apply.

Use the Right Tools!

This is the common mistake made out of negligence. The assumption that this tool will work good enough is a dangerous one. Using the wrong tools will make building the kit more difficult, and can make finishing an 80% lower a massive headache. Instead of listing all the wrong tools you can use, let’s list all the right ones.

  • Bits made for Metal. The drill bits you use to finish your 80% receiver need to be made for chewing through aluminum. I suggest buying these 80 lower bits from companies that actually sell 80% lowers.
  • Correct Router End Mill. Using a router to finish an 80% lower is an excellent way to go. However, using any old end mill is not a wise idea. There are custom end mills made for AR-15 80% lowers, use one of those.
  • Small Brass hammer – When it comes to installing a lower parts kit you need an impact device that is accurate. A small brass hammer is perfect because it is non-marring to aluminum.
  • Roll Pin Punches – Drive pin punches can work to install roll pins, but they run the risk of destroying a roll pin. A roll pin punch will not damage the roll pin, and is so much easier to use.
  • Castle Nut Wrench. Sorry, but a pair of Vice Grips will not work, unless you like a marred castle nut. A castle nut wrench is usually included in an armorer’s tool, which can be invaluable for an AR owner anyway.

Drilling Too Much

Drilling too deep into your 80% lower presents some really big problems. First and foremost this can effectively turn an 80% lower into a paper weight. Sometimes you can fix this is, but in some severe cases, it will be impossible to do so. Things to watch out for (But can be fixed)!

When drilling in the back pocket is you go too wide you’ll run into the selector detent hole. Measure twice, cut once, and take your time! Excessive cutting of the trigger area will weaken the receiver. If the Side Walls are cut too wide your trigger may slide left to right.

If you drill into the selector detent hole the best way to fix it is difficult. You’ll need something like Aircraft Grade JB weld to fill the hole. You’ll then have to drill out the selector detent wall, with the JB Weld acting as the portion you over drilled.

If you cut into the trigger area and fear weakening the receiver use some form of epoxy to sell in where you drilled. JB Weld Aircraft grade is an excellent choice to reinforce the walls.

Lastly when it comes to sidewalls and trigger shift, if you go too deep you may want to consider a drop in self-contained trigger. While they are a little more costly, they are often very high quality. They also won’t shift and can be shimmed into place.

Forgetting these…

Two things a lot of new 80% lower receiver makers will do is forget to use two invaluable milling aids!. The first is a stop collar, also known as a depth collar. Stop collars attach to drill bits, and are tightened down at a certain measurement. This prevents the drill from going deeper that required. One big thing to remember is to stop and check the stop collar on occasion. It may need to be tightened or readjusted.

Lastly, do not forget cutting oil! Cutting oil will make life much easier for you and will preserve your tools. Cutting oil lubricates as you drill. This makes the process easier and reduces friction and heat. Too much friction and too much heat will melt the shards of aluminum to drill bits and to end mills. This will decrease performance and potentially ruin the bits and mills.

Not Reading the instructions!

If you have never finished an 80% lower you need to take your time, and read the manual. If you don’t understand something don’t just guess. The internet is an excellent source to check, especially Youtube.


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