Matt, if youre talking about the MP9 then you should have that already under submachine gun.
Here yo go fellas, R6 upgrade list as you get promoted :
Private 1st Class - exp 3000: Headgears 1 (Tactical Helm, Balaclava)
Specialist - exp 8400: Rainbow Weapons (MP7A1, SIG552, USP40)
bCorporal - exp 14500: Camo 1 (Desert, Urban, Russian, Guerrilla, Fall, Desert 2)
Sergeant - exp 23500: Tactical Armor (Raven Recon Armor, Vulture Combat Armor, Falcon Assault Armor)
Staff Sergeant - exp 38500: Headgears 2 (3 Hole Balaclava, Baseball Cap, Breathing Mask)
Sergeant 1st Class - exp 56500: Camo 2 (Flecktarn, Orange, Swedish, War2k5, Alpen, White)
Master Sergeant - exp 82500: Freedom Fighter weapons (AK47, Raging Bull, SV98)
First Sergeant - exp 109500: Headgears 3 (Bonnie Hat, Beret, Tinted Goggles)
Sergeant Major - exp 139000: Black Market Armor (Typhoon Recon Armor, Cyclone Combat Armor, Hurricane Assault Armor)
2nd Lieutenant - exp 172000: Camo 3 (Pink, Blue, Woodland, Wasp, Sand, Crimson)
1st Lieutenant - exp 214000: Headgears 4 (Half-Face Mask, Reinforced Helm, Tactical Goggles)
Captain - exp 264000: Merc Weapons (MG36, SPAS12, Deagle)
Major - exp 330000: Military Armor (Vier Recon Armor, Diamondback Combat Armor, Anaconda Assault Armor)
Lt. Colonel - exp 415000: Camo 4 (Yellow Urban, Red Urban, Tiger, Rust, Urban 2, Grey)
Colonel - exp 535000: Headgears 5 (Ballistic Face Mask, Riot Helm, NVGs)
Elite - exp 675000: Camo 5 (Custom 1, Custom 2, Custom 3)