Thank you for all of your replies guys! Never thought this question would give such a reaction...
Looking at them i conclude that it is possible to sink a crippled ship with the flak gun but takes forever and does not garantuee a kill.
i will try it once it happens again and if that doesnt work i might just try to ram it to see what happens although i dont think my little duck will have much effect on a c3 or a juicy tanker...
Thanks again!
