Must weapon mode (piercing or sustained), other than defensive vs. offensive, be decided during declaration or when actually firing?
Standard, raking, and piercing mode don’t have to be announced during fire declaration, but you must announce them when you are rolling to hit (especially piercing mode, since your chance to hit is different). Sustained mode doesn’t have to be announced at all until the weapons fire step of the sequence of play is completed (at which time it will be bloody obvious that the weapon is still firing at the target, and hasn’t turned off).