SIG 556 Match Sear/Hammer Installation ShootingSight LLC 2012
Safety Warning Always verify your firearm is unloaded before working on it. These instructions are meant as a supplemental guide to an armorer or gunsmith who is already competent to install, adjust, and safety/function check a trigger and the components in a trigger group. They are not meant to educate someone with insufficient experience in working on firearms/triggers on all the practices necessary to correctly perform these tasks. If you are not competent to install, adjust, and perform trigger safety and function checks on a trigger and trigger group, do not attempt to install this kit yourself, but take it to a gunsmith. The ShootingSight match sear/hammer kit for the SIG 556 maintains the SIG trigger s adjustability for second stage sear engagement. Following installation of the kit, the second stage needs to be re-adjusted, so it functions correctly. Failure to correctly install and adjust the trigger can inhibit proper function, leading to a dangerous situation that can include accidental discharge, resulting in injury or death. This sear/hammer kit is designed to function in a trigger group that has other original SIG parts in it. Due to manufacturing tolerances, wear, or the possibility of modifications or trigger jobs having been done, proper functioning of the trigger must be assured via a function and safety check after installation and before the rifle is put in service. If any parts in the trigger group are worn, damaged, modified, or out of tolerance, these parts should be replaced before installing the ShootingSight match sear/hammer kit. Certain limits of adjustability are described in the installation instructions. Do not attempt to adjust the trigger below these described limits, as proper trigger function might be inhibited. Never modify the trigger or replace parts to the trigger group with non-original parts. This installation requires disassembling parts that contain springs under load, which could eject when released. Always wear safety glasses and other appropriate Personal Protective Equipment when working on firearms.
Installation Ensure weapon is unloaded. Tools needed: Small hammer 1/16 pin punch 5.5mm and 7mm open end wrenches or needle nose pliers. 5mm Allen wrench for grip.
Installation 1. Remove left side selector by removing roll pin using a 1/16 pin punch. 2. Remove selector shaft to the right, rotating and pulling it out. Note that there are two springs that are held under tension by this shaft: the safety detent spring (which also serves to lock the trigger and hammer pins in place), and the trigger return spring. Safety Detent Spring Trigger Spring
3. Remove trigger pivot pin and lift out trigger assembly. 4. Remove safety detent spring, which hooks over the hammer pin. 5. Remove hammer pivot pin and lift out hammer. Note that hammer spring is under tension. Safety Detent Spring Trigger Pivot Pin
6. Replace SIG hammer and spring with ShootingSight match hammer and spring. Note that the hammer pivot pin has a groove in one end this must be on the right side. 7. Replace the safety detent spring by hooking it into the groove in the hammer spring. New Hammer, Old Hammer
8. Disassemble trigger assembly by removing trigger hub to the left. Disengage small sear spring from the groove in the top of the trigger, and remove sear and plastic spacer. Plastic Spacer Sear spring Trigger hub SIG Sear
9. Replace the SIG sear, plastic spacer, and sear spring, with the ShootingSight match sear, the new sear spring, and the Teflon tube guide. 10. Orient spring so flat ground end is in sear, and the crimped coil is upwards to engage in slot in trigger body. 11. Place the sear in the trigger and re-insert the hub tube so it passes through the oval hole in the sear. Ground end is flat and goes in sear Ground End Slot in Trigger Opposite end has crimped coil to engage in slot in trigger.
12.Compress the sear spring, and feed the top edge under the trigger body. Be careful doing this so it does not eject if it slips loose. 13.The top end of the spring needs to engage up into the groove in the stamped metal trigger body. 14.Verify that the sear moves freely inside the trigger and that the sear spring is forcing it forward. Groove in trigger body. Top of spring can be seen where it has engaged inside the groove.
15. Add trigger spring, and re-insert trigger group in receiver. If the holes do not quite line up, press the sear back into the trigger slightly. Insert trigger pivot pin so groove is on right side. 16. Insert safety from right side. Use a small screwdriver to lift up the safety detent spring to pass over the safety, and press down the trigger spring to pass under the safety. Its easier to do this in 2 steps on the two diameters. 17. Twist the safety back/forth and push in until it fully seats. Verify that the safety detent spring has engaged both the grooves in the trigger pivot pin and the hammer pivot pin. 18. Re-install the left side safety selector. Groove in trigger pivot pin.
Adjustment There is a spring loaded plunger in the pistol grip behind the trigger, which is designed to contact the trigger prior to hammer release, so shooter feels a second stage of pressure just before the rifle shoots. Per the SIG Armorer s manual, this plunger should be adjusted so it contacts the trigger 0.5 to 1.0mm before the trigger releases the hammer. If this setting is misadjusted, the trigger could lose its second stage and the rifle could discharge without the shooter feeling a second stage of pressure prior to discharge. This second stage of pressure should therefore never be relied upon as a primary safety mechanism. If this plunger is adjusted outside the SIG recommended range, the armorer or gunsmith must assure that the trigger is functioning correctly and safely before putting the rifle in service. The design of the plunger and the contact surface on the back of the trigger are not precision surfaces, and the exact contact area can change slightly as parts move or re-orient slightly during use. Attempting to set the second stage plunger position so it just barely contacts the trigger prior to discharge could result in a dangerous condition where the trigger discharges prior to contacting the second stage.
1. Remove the pistol grip by unscrewing the 5mm Allen bolt inside the receiver. Depending on the model of rifle, there might be a detent spring for the takedown pin under the grip, which must not be lost. Detent spring
2. Using a 7mm open end wrench and a pair of needle nose pliers, adjust the position of the plunger in/out until it contacts the trigger the prescribed distance before hammer release. This is usually easier if the trigger guard is opened to swing it out of the way. 3. Re-install the pistol grip, being careful that the detent spring has not fallen out, and also that it feeds in to the hole, rather than bending and being crushed under the pistol grip.
Safety / Function check After reassembly, verify trigger has a second stage by cocking hammer and pulling trigger. Trigger must contact second stage plunger and you must feel the second stage pressure before hammer drops. Verify that after pulling trigger up to second stage, but releasing trigger without having dropped the hammer, trigger and sear return to full-forward position without hanging up or sticking. Verify disconnector function by cocking hammer while trigger is held back. Disconnector must catch hammer while trigger is held in back position. After cocking with trigger held back, check that trigger resets when released. There should be an audible click as trigger resets, and hammer must NOT release as trigger resets. Following this test, pull trigger to verify that hammer is released properly. Verify that safety is functioning correctly by cocking hammer, engaging safety, and pulling trigger to make sure hammer does not fall. After this test, disengage safety with finger off trigger, making sure hammer does not fall upon safety disengagement. Finally, pull trigger and make sure hammer does fall after safety is disengaged. If any of these tests fail, do not shoot the rifle, as it might be unsafe. Have a qualified gunsmith repair the defect before attempting to use.