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Avoid These Mistakes in Finishing Your 80 Lower

Posted by Jordan Vinroe on Oct 25th 2019

If you are new to transforming 80% lower receivers into a fully functional firearm, the process can be frustrating. To make sure your next build goes as smoothly as possible, avoid these common 80% lower mistakes:

AR 80% lower mistakes

Not Having the Right Tools

It might seem straightforward, but you need to invest in the right tools. Start by having the right mill bits for your drill that are specifically designed for the tough aluminum used in lower receivers.

The finishing jig is critical and should be purchased if you are just building off a straight 80% lower to avoid mistakes.  Full kits typically include a finishing jig. A handheld drill – or better yet a drill press – is also important. Make sure whichever option you choose can stand up to the materials of the receiver without breaking.

Misaligned Holes or Improper Cuts

Another common work mistake is holes or cuts made into the receiver that don’t line up properly. Since finishing receivers take a series of milling and drilling, a misplaced hole can potentially throw your entire build off.

Misaligned holes typically happen because an inexperienced builder will try to do all of the cutting with just a hand drill. Using a drill press vise can make a big difference in ensuring your receiver stays in place during milling. You must also be sure the guide plates are firmly tightened and in place before drilling your first hole.

Wrecking the Finish with Poor Drilling

Even with precise drilling for the holes, you can still make mistakes on 80% lower work. This includes rough cuts and uneven surfaces. For clean, precise cuts on your 80% lower, you need the right cutting speed.

Machining at 25,000 RPMs and adjusting as necessary is usually a good method. However, occasionally reducing speed is a good idea to eliminate chatter (when the end mill bit creates vibration) as well as excessive heat or runout.

Overdrilling the Receiver’s Holes

Perhaps the most serious 80% lower mistake to avoid is over drilling the receiver. With every hole you drill, make sure to read the instructions included with the receiver and measure twice before cutting.

 Woman Shooting Rifle

Not every hole requires the same depth for your firearm to function properly. Removing too little of the metal – or even too much – can cause parts of the receiver to weaken and ultimately result in a weapon misfire.

Overall: Don’t Panic

Even experienced builders occasionally make mistakes by not lining something up correctly or getting over-ambitious with their design – resulting in a damaged receiver. The key is to not panic if this happens. By following the instructions with your kit and avoiding the above 80% lower mistakes things should go smoothly for you.

Consult our experts at JSD Supply for tips to avoid mistakes and get your build back on track.