Trie iron's description is pretty close to what I've been taught, thorugh to enter with less then three is a no- no.
I would add lots of noise. Commands to "Police! Drop the weapon!" Sound a lot like "Please, drop the weapon!" to witnesses. The badguys just need to comply or get shot and a badguy who complies is unarmed and can't shoot you, whether you've had time to shoot him or not... The noise is also distracting and can be overwhelming if total surprise is achieved.