Regardless of all of those things, shooting even a low base #8 12ga round through a pistol is the proverbial "handful". Once upon a time I had a Thompson Center Contender chambered in 44mag/.410. For those not familiar, it is a very well made single shot handgun. Touching off a .410 shell was an adventure, a .44mag even more so. I believe the TC scope last less than a half box of shells before it shattered internally.

If it was life or death, the pirates were attacking and you held on tightly with both hands I suppose you could defend yourself with a flare pistol, I don't think it would be my choice though. I would guess you could do more to hurt yourself with such a rig than hurting the pirates?