First, I don't think you CAN kill a CV with 50cals. But even so the person that does the last gets it if I recall. Maybe it was a lag issue or a torpedo or some other reason you didn't get credit. You'd need a film to figure out why.
BadHorse, back when they implemented the points in buffer system, I upped Ju88s, flew around forever looking for a CV, couldn't find it, went and bombed 1-2 small bombs on a field (1 for dar, one something small like a bunker) then circled more, found the CV, dropped right on it, and got like 8 Million damage points when I landed later. They truncated the points shown after this, keep in mind, but it was still a surprisingly high amount.
P.S. I don't think guns damage counts. It's damage from bomb blasts only, and only on objects. You CAN damage an object without destroying it and land points. I've done that a few times with strats and such. Seems (to me) that the fire from your bomb touches something and you get points for that.