This is an extension of how Soldat handles the minigun over the network. Currently, according to MM, a packet indicating that you are firing your minigun is sent to the server and distributed to all other clients. You, the server and all other clients will spit out bullets with different pseudorandom sway until a packet indicating that you have stopped firing is received. This method is used to prevent the minigun from flooding everyone with hundreds of "bullet" packets, due to its high rate of fire. The drawback is that the bullet spread you see isn't necessarily the same as that the server sees when checking if bullets have hit other players.
To solve this, the player that sends an "I'm shooting now" packet should also send an integer (provided by some tick/millisecond counter) that will act as an initial seed to be associated with that player when generating the pseudorandom sway of his minigun's bullets. The server and every client would thus be able to generate the same sway.