The M249 LMG is the best all around weapon in the game. It can quickly and easily destroy vehicles and armored cultists. It is faster than a rocket launcher, and it has much more ammo. Immediately unlock the Heavy Weapons Mastery and Vehicle Buster perks for it to make it much better. Additionally, use armor-piercing ammo to easily destroy the strongest soldiers and helicopters. A quick burst from the M249 will destroy everything in your path. There is almost no reason to equip other guns. It is like an automatic bazooka.