The Fire control group (FCG) on the left is an unaltered original group from a de-milled parts kit. The group on the right is our semi auto configuration.  We have replaced the original sear with our US made component. This eliminates the follower leg or lobe which works in conjunction with the original bolt for full auto capability. To stay within the acceptable limits of federal law you must be sure you do not have ALL components required to make a “machine gun” in one place. It is good practice to be aware of all the parts on hand at any given time. Disabling the FCG before staring to build your receiver is advised to avoid any NFA issues.

Sear Parts

Our part above has an angled sear engagement in the upper right corner of the photo. Other enhancements allow it to seat at the proper height in the FCG. (Detail photo below )

Triggers

Your original trigger shown above needs the extension removed as the example on the right. Be careful not to remove more than just the extension. Your final measurement should be as shown in the next photo.

Also note the modified part above is ground slightly on the lower portion shown in the white. A thickness measurement is shown a few photos below.

Trigger measurement

This photo also shows the Radius ground on the exposed face. Again be careful not to remove anymore from the length shown above. A good starting point is as long as possible. An Angle back works in place of a radius and may be easier for you to do.

 

Sear height

This distance measured above is important to allow for the trigger to reset properly. We have found good results around a range of .130  If your radiuses are ground evenly on your modified parts things should operate smoothly. We will show another radius part you will need to do as well below.

set screw

We recently changed  this set screw to 5-40 from 6-32 above shown on the left side of the  FCG for tighter control of the height of the sear. Fine adjust to maintain the .130 -.140 measurement needed. Use red lock tight to secure it.

As of 8/11/19, we have also added a 5-40 nut with this to act as a lock nut to insure the position of the set screw remains fixed. This is critical to the smooth and safe operation of the FCG. The nut is placed on the inside of the group.

Trigger thickness

+ - .003 therabouts... the thiner it is, the more take-up you will have with the trigger.

Following photo shows the second part that needs modifying. The tiny spring loaded disconnector needs to be rounded on the bottom edge as shown with the arrow. This rounded side engages with the rounded top portion of your modified trigger allowing it to reset. Do not remove any of it's lenght when shaping.

Try and strive for a .030” overlap or sear engagement with the trigger. If you’re coming up too short or feel the pull is too light then you can go deeper with the counter bored hole in the sear itself. Use a #3 drill to lower the depth as needed. If you aren’t getting any movement doing this you may need to bore the through hole with a .154” reamer (because the flat is holding back the advancement). **Do not bore all the way through. Leave a slight amount of flat on the exit hole to keep the disconnector indexed properly.

disconect

For ease of assembly we tap the holes that mount the FCG to the flanges on the receiver. The existing rivet hole is the right dimension to run an 8-32 thread tap right into it. A ¼” long button head Allen screw works well for this on both sides.   

Taping

PLEASE NOTE * That the radius grinding and dimensions listed above are a starting point. If you have too much sear engagement remaining you may run the risk of the gun double firing if it has not disconnected when the trigger is trapped. Fine adjustments on each grind or set screw will get you to the right amount needed for a timed disconnect. During the reset of the trigger make sure the two surfaces push through each other easily. A dap of oil on the radius points goes a long way for this.

 Trigger Pin

We found we needed to make new trigger pins with all the in and out we had to do during the process. They are available in the parts section here on the site. Also available is the 8-32 set screw we use to mount the FCG