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"Is there any functional difference between a "maverick" and a "loose cannon"?"
Yes, they are similar but different
A "loose cannon" is an unpredictable but VERY DANGEROUS person or thing.
A "maverick" is just an unclaimed steer, or wild unclaimed thing of some sort. A maverick is also a slightly unusual thing, but it really means more of a "odd duck" or the like.
The loose cannon has the connotation of danger.