I don't understand why the M99 wasn't added to the game as it was in the IJC weapons pack.
Exactly. M99 ain't no overpower in IJC pack because it is SLOW.
It is a cumbersome weapon. Slow to swap, slow to aim-down-sight, slow to reload and ... ALL weight. Anti-material snipers who does nothing but taking out the big ones with a slow rof. Similar to LAW in demo's hand.