Having looked at the bear in the above thread, I think the following may be in order. Not all of which would be entirely practical in the cockpit...
Belt-fed version of the HK 51
An old favourite which should handle most things, but fairly expensive to run, and...
The .700 Nitro-Express. Will take down anything, period.
You may want to check with local authoritys as some of these firearms may have some restrictions in your area.