Using a drill press or mill w/a screw driver blade set in the slot and held securely down and then turn the quill by HAND as DerAmi suggests is the usual final way to success on the stubborn ones.

I've had the bit's break in attempts to remove some screws in this way but ultimately being successfull. At least there's no chance of slipping/skipping out of the screw slot with the bit or doing a wriggle pattern with the screw driver down the side of the frame.

The bushing may already be broken in 1/2, and with the torque applied by the blade inserted in the slot and pushed down, it is separating the halves and wedging them even tighter in the threaded hole.
The more pressure,,the tighter the grip the bushing has in the hole.

Take a moment and with a very small, thin narrow screwdriver blade, place it down onto and slightly into the slot on the one side of the bushing firing-pin hole so that a gentle tap w/a small hammer will show if it will rotate/unscrew at all.
A little oil in and around the bushing will help show if there's any movement of the part.

You don't want to damage the screw slot of course,,but you are just trying to see if the bushing unscrews or moves at all with the little help that this gives it.
This will not put any of the wedging action a full screw driver can produce to a cracked bushing, if in fact it is and we don't know that.
It is just to find out if the bushing is being locked in place by the common removal method.

If it moves at all with this, then a far move light handed approach to removing it is needed. Kind of kicking it around in rotation a part turn at a time with the same technique till it's out far enough to unscrew w/your fingers.

...and maybe it's just totally rusted into place and ain't gonna move w/o breaking.


Is there a real need to remove the bushing?.
Is it damaged or pitted, in need of replacement?

I know the feeling of 'it should come out,,it's made to be removed".
But in this case, removing may mean damaging. You'll win the war by removing it, but may wreak it in doing so and now you have to make and fit a replacement.
Hardly worth the exercise I'd think at this point.