It took me some looking around for this info which is really dumb BUT to use your alt-fire on your weapon in multi-player is...
Hitting L1 + R1.
Hitting both at the same time should toggle between your regular firing mode and your alt-fire. I recommend doing it after you used your grenade though.
Hitting L1 + R1.
Hitting both at the same time should toggle between your regular firing mode and your alt-fire. I recommend doing it after you used your grenade though.