i made a couple tables about crits. you can find the infantry crits here:
https://docs.google.com/spreadsheet/ccc?key=0ApmrrrPr20ncdEpuSHcxNko1VGVFYjczYXpFZWhqOHc&pli=1#gid=12
and vehicle crits are here:
https://docs.google.com/spreadsheet/ccc?key=0ApmrrrPr20ncdEpuSHcxNko1VGVFYjczYXpFZWhqOHc&pli=1#gid=11
but to answer your question, crits are all based on the type of damage. so these instant kills are only for flamethrowers. explosives can cause a crit similar to the flamethrow though. it cant crit entities with >50% hp, but <50% hp has the same 10% chance to die. small arms and ballistic (tank guns) damage have no crits.
