Due to the cost of purchasing a new weapon, it may be more economical to upgrade a current weapon. Some missions require you to use specific weapons. Do not sell all your extra weapons; try to keep at least three of each type at all times for each crew member. Rare and epic weapons are the best available because they do the most damage. However, they are difficult to optain. You can get them for free during the gun rewards after missions are completed. They can also be purchased from the shop when you have enough money. You can increase the rarity of a weapon by leveling it up.