Problem is the radius of the hammer lob over the top of the sear on the hammer.  


I could write a long winded version of solving that, but the easy way for you to solve the problem, is to just lightly radius the back edge of the selector cut out flat so it can glide past the tail of the trigger when the selector is switched and the hammer is held all the way back via the B/C.  

Or, just hold the trigger all the way back so it's tail is against the disco tail with the upper shotgunned open, and switch the selector a few times to allow the tail of the trigger to round the selector flat edge isntead.

As for the hard selector switch with the B/C all the way back, your not doing any harm with it being stiff now, and in time like you holding the hammer back and doing the selector flips, this will not only round the selector edge in question to allow it to pass by the trigger tail with less binding, but may end up peening the top of the hammer lob radius to correct the problem as well.

Just the trailing edge only on the selector from the flat cut, to the rounded sections.

The main part of the flat is where the trigger needs to max out against the trigger is pulled so it does not over travel after the hammer is released, and the main flat of the rounded surface is where the selector needs to block the back of the trigger from rising when the selector is set to safe and you try to pull the trigger.


Also, leave the back flat of the trigger alone!!!!!!