M2 trail is used for M2 (who would guess
), M79 and grenade projectiles.
As Espadon said - to make rainbow look ok with rocket flyin' in every direction, smoke should be made as circle (like bowing target?). Also smoke can't be too big as it's showed in game from left top corner and it would screw up with bigger size.
'Bout sound - no good method, if it would be in fire sound slot, as I remember well, it'll be played as long as you'll hold fire button, pre-shoot idle sound is played till shoot.