I had a request to see if it was possible to have a two-stage trigger, like what you find on P90s, F2000s and AUG guns, on a recent M14 PolarStar Fusion Engine conversion. With a two-stage trigger, a partial trigger pull shoots a shot in semi-auto mode, and a full trigger pull shoots in full auto. Usually the M14 Fusion Engine conversions have the fire select implemented used a slide switch added to the body, or with the G&G M14s, a microswitch is added to the existing selector switch. With the flexibility of the Fusion Engine trigger and fire select functi onality, it is possible to add two stage trigger capabilities to guns that use the traditional AEG trigger switches.
The relatively simple modification to have a gun shoot in two-stages is to make use of the trigger switch center trigger pin, and then stagger the trigger contact blades. The Fusion Engine uses a common ground wire on the trigger circuit board. By wiring the center trigger pin to the common ground, then wiring the trigger wire to the longer blade, and the full auto wire to the shorter blade, the trigger pull will fire in two stages. It may require a little tweaking and testing to make sure the center pin cleanly contacts each blade and the trigger pull is smooth, but it appears to work very well and it adds a new dimension of fire mode capabilities to Fusion Engine guns.
An additional modification that is possible on the M14 is to use the fire select switch to enable/disable the full-auto functionality by having the fire select switch wired in a series with the full auto wire. This will allow the gun to shoot in semi-only mode, and two-stage mode.