Seeing as how there are variances in how slides are countersunk for the front sight posts on "C" (and "E") series guns, I thought I'd put up some diagrams in regards to what I've found is involved with "fitting" the Trijicon front night sight. YMMV.
First, do a dry run installation.
Check that the sight will fit flush on the slide. Then check the posts for correct clearance on the inside of the slide. Finally, install the screw and check that it holds the sight flush to the slide and fits correctly into the countersink.
*note -
in the diagrams, fit of the sight posts in their holes and screw head in the countersink are exaggerated to show the parts clearly. The posts fit closely in real life and the screw head will be in contact with the sides/bottom of the countersink.
Once satisfied that the fit is correct, install using an adequate amount of red Loctite.
Post #27 contains instruction for removing the factory front sight and installing the Trijicon sight (some mention in post #1 as well).
Regards,
Greg