SIG 55x DMR Trigger Installation

SIG 556
Match Sear/Hammer Installation
ShootingSight LLC © 2012
Safety Warning
• Always verify your firearm is unloaded before working on it.
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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.
2.
Remove left side selector by removing roll pin using a 1/16” pin
punch.
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
SIG Sear
Sear spring
Trigger hub
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.
Slot in Trigger
Ground end is flat
and goes in sear
Ground End
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.
3.
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.
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
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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.