first guess is a cracked selector plate, right at the arch...
if it is cracked then when selector switch attempts to move to Auto
it will not pull the plate to shift it properly to full Auto position
(basically fully lifting the Cut Off Lever upwards 101%)
Sounds like the COL is not raised fully so trigger trolley is hitting the COL
than slip under/over it
Though I would have thought the trigger would only pull say 30% of travel before hitting the trolley/COL
Could be trigger contacts impeding the trolley, but initial thoughts are selector plate
Did you remove box recently, usually plates can break when refitting box into receiver
than break on there own
Though a stiff selector or stiff plate, catching rather than moving freely can wear/break if forced
or also if gun is abused or dropped - stuff happens
but broken/cracked selector plate is my first guess
If you do replace a faulty plate, ensure you make close comparisons between old/new
some plates, the arch appears in a slightly different place which if not checked can also create issues
where the COL is not fully raised/lowered or safety doesn't fully operate properly in all 3 positions
Usually they are fairly compatible, but they can/do vary if you replace genuine parts with aftermarket ones
if semi is working correctly, then trigger trolley and trolley stop pin is all intact
so this would likely point to cracked selector not fully raising the COL for AUTO to function correctly
reckon cracked at arch, then it doesn't shift the whole plate as it moves back so COL not raised on Auto...
It isn't an ambi fire switch is it ???
don't think so, but just checking in case other selector switch has gone tits up on other side
and impeding the movement on other side/plate shifting selector plate to auto properly
(doubt it, but there is some reason why plate isn't operating properly to move to Auto)
