When you're ready and when you're not. If your attack is going well, you've just walked into an ambush. Anything you do can get you shot, including nothing. Make it tough enough for the enemy to get in and you won't be able to get out. When you have secured an area, don't forget to tell the enemy.
All fights are unfair.
It's just a question of if it's unfair in your favor. Don't think of as being outnumbered, think of it as having a wide variety of things to shoot at. It's not self defense if you reload twice. You are a pacifist. You just need the assault rifle for self defense.