Have the Right Gun Equipped To Deal with Where You Think Danger May Come Out You Quickly
This is similar to the point above, and generally only comes into play when you are moving. Fights can last less than a second, and you can be killed before you can switch our your gun to a better one.
When danger can be right around the corner, have a short range gun out. When it may be over a hill, have a long range gun out. You might find yourself switching gun often as you move about. Thats fine.
Take That Silly as Scope Off Your Rifle When the Circle Is Small or You Know Your Opponent Is Close
This is a mistake I make often, and feel very smart when I remember not to make it. The iron sight for rifle is fine for medium to close range combat. You can also enter ADS mode fast when you aren't using a scope.
You are going to feel really stupid trying to aim your carbine at someone at 10 meters with a 6X scope. You can put the scope back on when you leave town.