[TXAZ]

Ditto to the several 'it all depends'. And like military plans, nothing goes as planned for either side.

If you work in the same building as or next to the SWAT team, hunkering down for 1 minute is probably the best option. They probably have a more certain shot through the windows than I would.

If you're Mano-a-Mano, it's probably rapidly engage-or-die.

If it's a disturbed person who demands to read his 25 page manifesto first while everyone listens, it might be worth waiting unless you've got a clean, clear shot.

If as the Israelies sometimes encounter, someone who says they won't negotiate, someone is likely to depart the world in the next few seconds if they're on-scene.

Finally, if the shooter also has a cellular trigger with a deadman switch (and that's not only in the movies) its a very tough call.

It all depends, and I can't imagine any handbook that could possibly classify paths to the best possible outcomes for every eventuality.