Shimano BR-RS505 brake caliper retainer screw stuck My bike has a set of Shimano BR-RS505 hydraulic brakes and the rear pad axle, datasheet item 10, just won't budge.

The flathead screw stripped a little when I tried to release it and I didn't go any further.

My first thought was to carefully apply some WD-40 (all I have) into the screw then use a heat gun to hopefully loosen it off.

I've also been told that the only other solution would be using a small hacksaw to cut through the pin!

How should I proceed? Thanks!

(I have also bought replacement, seemingly compatible, hex screws rather than flatheads..)
The axle pin is a combination of threaded (near the head) and smooth (body that the pads move on).  It would be hard to replicate that part without removing the threading from a hex headed bolt.  

To start at the beginning, did you remove the retaining pin before attempting to unscrew the axle/pin?  There is a cotter/retainer at the other end of the axle that would prevent rotation if it wasn't removed first.  

If all that has been removed and it is still stuck, I would remove the caliper from the frame and try some break free or similar to free the stuck threading.  This will ruin the pads (likely), but you are likely also trying to replace them.

If you use a hacksaw to cut the pin, the threaded part will likely stay in the caliper body.  If anything, on my own brakes I may use a cut off wheel (like on a dremel) to deepen the slot for the flathead or to make a cross cut and allow Phillips use.  After doing either, I'd likely start looking to order a new pin and retainer (part 9) and perhaps look at cleaning of the threading inside the caliper body.