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We are a national retailer of individual components and not all products depicted on this website are legal in every state. Shipping of various products found on this website are prohibited to some states (such as California, Connecticut, New Jersey, New York, Pennsylvania, and Washington). The information, pictures, text or products presented on this website are not a representation by us, and should not be understood by you, that any product or completed firearm is legal to assemble or own in your state of residence. We encourage each and every builder to perform their own research about the state and federal laws that apply to them. It is your responsibility to understand the law and we encourage you to consult with an attorney or your local ATF representative.

Polymer JMT 80% Lowers: Machining Instructions

Posted by 80-Lower on Aug 10th 2020

Polymer JMT 80% Lowers: Machining Instructions

James Madison Tactical's polymer 80% lower includes the jig with templates for successfully cutting and drilling the receiver to make it functional, ready for a lower parts kit install. Before we get started, you'll need a number of bits and tools. These instructions are applicable for the JMT polymer .308 lower, too.


Drill & End Mill Bits Required

  • 3/8" Drill Bit: This bit is required for drilling the safety selector lever pin hole.
  • 5/16" Drill Bit: This bit is required for drilling a pilot hole for the end mill bit.
  • 5/32" Drill Bit: This bit is required for drilling the hammer and trigger pin holes.
  • 5/16" End Mill: This bit is required for milling the fire control cavity and cutting the trigger slot.
  • 3/8" End Mill (Optional): A larger 3/8" end mill will cut the fire control cavity more quickly.

Tools Required

  • Drill press or mill: You'll need either a drill press or a milling machine to cut and drill with.
  • Vise: A vise is required for securing the jig and 80% lower together for machining,

Optional Tools

  • Calipers: Calipers will aid in verifying you're cutting the receiver to the right depth.
  • Drill Stop Collars: Stop collars can be used on your bits to prevent cutting or drilling too deep.
  • Cutting Fluid: Cutting fluid will reduce heat and produce a cleaner finish when completing the receiver.

Machining Instructions

Before getting started, you may download a .PDF version of the JMT jig's instructions. During these machining steps, you will complete three major tasks: 1.) You'll drill the pin holes for the hammer, trigger, and safety. 2.) You'll cut the fire control cavity to install the lower parts kit. 3.) You'll cut through the floor of the receiver for the trigger slot.

1.) Secure Lower and Jig

Secure the 80% lower inside the jig's two halves.The pieces should snap together with the lower clamped inside. 


2.) Clamp and Drill Pin Holes

Next, clamp the lower and jig together in your vise. Orient the assembly horizontally, so the right side of the jig and receiver are facing up. Before drilling, you must ensure the jig and receiver are perfectly square with the drill bits in your drill press or mill. You must drill three holes into the receiver:

Use a 3/8" drill bit to complete the safety lever pin hole, and use a 5/32" drill bit to complete both the hammer and trigger pin holes. Drill completely through the receiver via the pin hole guides in the right side of the jig. Drill in steps, frequently retracting the drill bit to clear each hole of debris.


3.) Set Up Receiver for Cutting Rear Shelf

Once all three pin holes have been drilled, the receiver and jig can be loosened in the vise. Flip the assembly 90 degrees and re-clamp the jig and receiver vertically in the vise. You'll need the 5/16" end mill for this step. Attach the included template plate atop the jig for cutting the rear shelf. Tap the plate onto the jig until fully seated. As shown above, you'll be cutting within the rear guide of the template, where the buffer tube housing and rear shelf area reside.


4.) Cut Rear Shelf Area

Cut to a depth of 0.630" at 600 to 800 RPM. 


When measuring depth, measure from the top surface of the receiver, not the jig or template. A cross-slide vise is highly recommended to ensure easy side-to-side movement is provided for cutting efficiently. It is recommended you make shallow cuts of an approximate 1/8" depth per pass, especially if using a drill press and not a milling machine. Frequently clear the receiver and end mill of debris. Once the rear shelf area has been cut, remove the top template plate.


5.) Cut Main Cavity

Cut to a depth of 1.250" at 600 to 800 RPM. 


The top template will not be used for this step. Instead, cutting the main cavity will be guided by the top of the jig itself. Use the 5/16" end mill for this step, or a larger 3/8" end mill if you want to complete this step quickly. Remember to measure your cutting depth from the top of the receiver itself, not the top of the jig. Continue cutting with 1/8" passes and frequently clean the receiver of debris.


6.) Cut Trigger Slot

Cut through the floor of the receiver at 600 to 800 RPM.


Reinstall the top template for this step. Using the small slot-shaped hole in the center of the template as a guide, cut through the floor of the lower receiver to create the trigger slot. You'll need the 5/16" end mill for this. If you're verifying your depths or measurements with calipers, the slot should measure 0.680" wide and be placed 0.662" away from the front of the main cavity.


Your JMT Lower is Complete

Once the trigger slot has been drilled, your lower receiver is complete. Now you can install your AR-15's lower parts kit before assembling your new rifle or pistol with an upper. This instructional guide covers how to install the LPK.

DISCLAIMER: If you are new to the world of DIY gun building, you likely have a lot of questions and rightfully so. It’s an area that has a lot of questions that, without the correct answers, could have some serious implications. At 80-lower.com, we are by no means providing this content on our website to serve as legal advice or legal counsel. We encourage each and every builder to perform their own research around their respective State laws as well as educating themselves on the Federal laws. When performing your own research, please be sure that you are getting your information from a reliable source.

We are a national retailer of individual components and not all products depicted on this website are legal in every state. Shipping of various products found on this website are prohibited to some states (such as California, Connecticut, District of Columbia, New Jersey, New York, Rhode Island, and Washington). The information, pictures, text or products presented on this website are not a representation by us, and should not be understood by you, that any product or completed firearm is legal to assemble or own in your state of residence. We encourage each and every builder to perform their own research about the state and federal laws that apply to them. It is your responsibility to understand the law and we encourage you to consult with an attorney or your local ATF representative.