1.
If you ~ a shot, you send a bullet or other missile from a gun.
A gunman fired off a volley of shots into the air...
...an illustration of a guy firing a huge cannon off into the distance.
PHRASAL VERB: V P n (not pron), V n P
2.
If you ~ a letter, question, or remark, you send or say it very quickly, often as part of a series.
He immediately fired off an angry letter to his ministry colleagues...
PHRASAL VERB: V P n (not pron)