The first tool you should have is the blueprint for an AR-15 receiver. Here's a PDF of the blueprint: ar15_receiver.pdf
The FCG pocket is divided into 3 sections: rear takedown lug, selector, and trigger assembly.
The rear takedown lug portion is 1/2" wide, .630 deep, and .950 long
These dimensions are basic fractional dimensions, with clearance added. To roughly relate them in fractions, here they are:
takedown: 1/2" wide, 5/8" deep, 15/16" long
NOTE: these are only for reference, refer to the drawing for exact dimensions and tolerances
To complete an 80% lower receiver we recommend the following tools:
The 3/8 end mill is used for the primary machining of the FCG pocket. A 4-flute center cutting end mill will be the easiest to use for plunge milling the majority of the pocket. Plunge milling is like drilling a hole, but you generally only overlap the plunges 1/2 the width of the cutter. This is a very fast and accurate way to remove material. Using a depth guide such as a piece of tape wrapped around drills and end mills is advisable.
The 5/16 end mill is for milling the hole the trigger drops through. You can simply plunge mill this slot and it will make the cleanest slot.
The 7/16 drill bit is for drilling the radius of the FCG pocket where the selector goes. The area where the selector is only measures 7/16 wide, using the drill to create the transition from the .630 deep section of the pocket to the 1.249 deep section of the pocket, is the easiest for the home builder. You can clean up the floor with the 3/8 end mill.
Similar to the method above, you can use a 1/2 drill to rough out the .630 deep pocket where the rear takedown lug of the upper receiver nestles. This pocket is 1/2 inch wide and you may need to plunge mill the corners to give them a 3/16 radius to clear the rear lug.
The remaining drill bits are for the various detent holes, slide lock spring hole, and slide lock lug roll pin hole.
When fitting an upper receiver you may need to file the front and rear takedown lugs on the upper receiver. The front takedown lugs on the lower receiver have a 1/16th inch fillet radius in between the lugs. Your upper receiver may already have a radius on the back side of the front lug, if it does not, simply file a small radius and fit the lug to the receiver.
The rear matching radius of the upper receiver must be machined all the way to the rear takedown lug on the upper receiver. Your upper receiver may have a small step where the upper was not completely machined. If this is so, the upper receiver will not shut completely on the lower, look for a small step and carefully file it away to fit the upper to the lower.
Since the end user must machine the FCG pocket, we have no control over the dimensions, but when properly machined to the blueprints, your upper receiver will fit our 80% lower seamlessly.