Clear the single player campaign on the following difficulties to unlock the associated weapons for the campaign:

Ensign
SEAL Weapons

Lieutenant
Terrorist Weapons

Commander
SBS Weapons

Captain
GROM Weapons

All Difficulty Levels
IW-80 A2
STG 77

 

Clear the single player campaign on Captain or higher difficulty to unlock the Foregrip and Medium Scope accessories.

N. Africa Terrorist ... Clear the North African mission.
N. Africa Terrorist Ghillie Suit ... Clear the North African mission on Commander or higher difficulty.
N. Africa SEAL ... Clear the training mission.
N. African SEAL Ghillie Suit ... Clear the North African mission on Commander or higher difficulty.
European SEAL Ghillie Suit ... Clear the Polish mission on Commander or higher difficulty.
European Terrorist Ghillie Suit ... Clear the Polish mission on Commander or higher difficulty.
S.E. Asia Terrorist ... Clear the Southeast Asian mission on Captain or higher difficulty.
S.E. Asia Terrorist Ghillie Suit ... Clear the Southeast Asian mission on Commander or higher difficulty.
S.E. Asia SEAL ... Clear the Southeast Asian mission.
S.E. Asia SEAL Ghillie Suit ... Clear the Southeast Asian mission on Commander or higher difficulty.

 

Weapons decrease your character's movement speed. Take only what you need or being weighed down with RPGs and other heavy munitions makes you a slow moving tank but without the Chobham-reactive armor. Smoke grenades are probably the only weapon you can equip in abundance that will not degrade movement.