Criteria for Weapon Selection
There are two ways of selecting weapons in common first-person shooters; the players can select their weapons before the game starts, and during the game, they get the opportunity to switch weapons. The theory is similar for both scenarios, but we'll cover the harder option of in-game weapon selection.
Two essential qualities are expected from the weapon-selection behavior, both contributing to the illusion of realism.
Humans can make many wrong choices during combat. Even if the weapon selection isn't ideal, however, a human player will generally stick with it. The last thing we want from the AI is to repeatedly change weapons, thinking it would provide slightly more optimization.
Changing weapons is quite visible externally, because the weapon is visible. One also really notices the change of projectiles more than anything. If the weapon selection is not working well, it will be noticed almost immediately from a distance. This really needs attention to maintain the illusion of realism.
This is essential to maintain the illusion of intelligence. It's a safe choice if the player can claim, "He had no other weapons, so he used the machine gun," or, "She used the blaster to fool me into believing she was out of ammo!" With the AI being constantly under focus, there is very little leeway. With one stupid decision (for instance, using an axe at a distance), or an exotic one, the credibility is lost. Advanced tactics can also attract unnecessary attention, so keeping it "standard" can help.