Still having the 'C' issue with unspent rounds? I had a similar problem in the past with a SIG P229. The SIG ejection port in the slide is very small. When loading long, it was possible to have rounds feed, chamber and shoot just fine, but have the bullet nose get caught up on the squared off inner edge of the ejector port when attempting to clear an unspent round. My bullets were all jacketed, so they'd be trapped until they fell down through the empty mag well. I'd say load some short and see if the C disappears or looks different. You might also try reshaping your ejector for a higher or lower trajectory or later ejection. Just a thought. YMMV, etc...
Or you could go all dremel on the slide... Post those pix!