Barrel over the line, trigger behind it pretty much defines where the dangerous bits go (and lets not start arguing as to where the barrel ends and a silencer/muzzle-break/flashy bling starts or stops).
I now shoot FT and the front foot can be over the line these days following a sensible rule adjustment i.e. barrel/trigger as above.
I have shot HFT courses where despite my being in the "safe" position (prone, on peg, etc) I have had to warn a shooter in the next lane that my gun was rather close to his head/ears with us both being in the right positions - not an ideal course layout on that day :-) Setters need to consider the lanes either side of the one they are banging the pegs in for.
Sounds like it is all about attitude - think safe, be aware of what and who is around you and when in doubt shout "Marshall!".