Conditions where you will see the armor icon if it absorbed anything: explosives of any kind. primary weapon damage without the AP perk. If you have AP then you get the 'x'.
For the crossbow, it has the same 1.4 head and helmet damage modifier as the smg. The crossbow itself, without the modifier, does 70 points base damage. It is only matched by sniper rifles. That plus the 1.4 modifier = 98 points. Few weapons in the game can kill in one shot to the head unless you use stopping power. Always been that way since 2007. Certainly players without armor are dead. No buts. However 30 health + 70 armor (or put another way health + non-regen juggy) is enough to stop one round.