any gun with high mobility, high accuracy and a high rate of fire. so basically you're looking at the submachine guns almost exclusively. the only assault rifles i would use on shipment would be ak47 or g36
for guns try any of these with a silencer (silencers work great on shipment because with so many people, a lot of the time players will blindfire at wherever they see people shooting on radar)
mp5 no attachment or silenced (any of the attachments ESPECIALLY the red dot will change the damage modifiers and actually make the gun do less damage)
p90 (rate of fire is crazy so do your best to conserve ammo, make a habit of laying down in grass or taking very good cover to reload)
ak74u (relatively high damage, pretty good accuracy. some people don't like the kick on it but if you get the hang of it it works great)
scorpion (very accurate, great to hipfire with)
mini uzi (very high damage at close range, good hipfire accuracy)
for perks try:
smoke (you'd be surprised at how well it works when you're about to engage a few people at your front, toss a smoke behind you and it will help against potential flankers while you're busy)
or if you can get past the "cheapness factor" 3frags with sonic boom as your third perk (trust me, the rules do change for shipment, it gets so hectic that there is more of an 'anything goes' mentality. whatever keeps the enemy team disoriented and at a loss for consistent kill streaks needs to be done. if it's a matter of catching a lucky grenade kill or there being an enemy chopper killing your whole team for a minute, which would you prefer?)
stopping power or uav jammer (always very very important on shipment to be able to do as high damage as possible. the quicker you can drop your enemy, the better your kill death ratio will be, or the less likely you will be to go negative

uav jammer works decent as well because as aforementioned, with so many people killing, radar will be up for most of the game and players tend to blindfire wherever enemies are appearing on the mini map)
steady aim or deep impact (if your gun is very accurate, use deep impact to help you get those double kills when players line up one after the other, if your gun is less accurate but high damage, slap on steady aim)