Easy problem to fix, and its the hammer lob  radis above the sear the problem on the hammer.


With the selector in fire, the trigger tail can move upwards as the hammer is being over cocked, and this allows the front sear of the trigger to come down farther without binding as hammer moves to the radius tab above the sear of the hammer.

With the selector on safe, tail of the trigger can not move up farther when the hammer lob radius above sear is not in spec, so the bind of the two happens at the trigger front sear to hammer lob radius at over cocked instead.

So to fix, radius of the numb above the sear on the hammer need to be lengthened, and in some cases, reduced as well.

Watch the below and focus in on the front of trigger and the lob above the sear on the hammer.




Also, the SP1 uses a L cut hammer FP contact face, and in some causes, it's the top of the hammer contact pad on the hammer (bottom of L cut edge) that needs to be lightly chamfered as well.

So L cut hammer on the right, and its the bottom of L cut edge on the top of the firing pin contact pad than can be a sharp enough edge to hang up on the bottom of the carrier as the carrier is moving back forward.  Nothing special needed, and a stone works fine to soften that sharp edge.


Short of the above, pull the bolt off the carrier and use the carrier with key in the upper receiver to check the gas tube alignment to the key.  The gas tube starts to enter the key when the carrier is about an inch from touching the front of the barrel extension, and if the two are binding up, will cause all kinds of problems the last inch forward of B/C travel.  If the gas tube is not indexed correctly, then pull the hand guards to tweak the gas tube over the center of the barrel to align the back end of the gas tube correctly to the key.

Just a FYI, but FCG pin size in a SP1 receiver should be .171" pins channels, with .171" FCG size parts.

Standard mil spec size for other receiver FCG's is .154" instead.

So before you start dropping in different FCG groups, make sure that the receiver pin channels are .154 in the lower receiver, and not the larger .174 that requires the larger pin size FCG instead.  

Hence you install a .154 FCG in a .174" pin channel receiver and the rifle is going to have nothing but problems.  And is easy to spot the mistake, since the .154 pins are going to be loose as hell in the receiver channels.