Mike
I imagine how you could have been disappointed. A 15 footer isn't that frequent, approaching him is a tough challenge. A pity you only had a double rifle. I am quite sure that with a scoped rifle, the croc hide would ornate your home. You should have been really pissed? That's the risk with the croc, any shoot out of the brain or the spine would have the croc sinking and being lost for ever. 1 cm could made all the difference.
I know what You mean with a sore shoulder when prone shooting.
The last day of the hunt prior to shooting the hippo and the croc, Buzz stopped on a flat island and tested my ability with his client 375 Blaser. Prone shooting at 100m on an empty pack of cigarettes. My shot went in the center of the pack,...........and my clavicle was toast. Buzz to be extra sure that it was not a lucky shot asked me to shoot one more..........same result.........and the gallant ultra-light 375 Sauer once again tatooed my clavicle. Not the best medicine to begin with the hunt. On the other side I was real bolstered.

500
I can't tell that I am an excellent shot, just often a good shot, ever more when I am determinated, what was the case the last day. In French we call it "energie du desespoir" (desparation energy) a combination of desperation and determination.
Look at this clip from this same hunting trip. How can one botch so easy a shot

failed bushbuck