all the above.
Head shot in Multi = Insta-reap.
remember that you are only wearing a helmet in multi that is not necessarily part of the nanosuit. You're wearing an economy version compared to the campaign suit.
Also, about the 40 shots. There are a couple of things to consider. Were they in armor mode? Did they have an armor multiplier perk? (gawd-aweful tank mode.
)
And then there is lag and/or packet loss:
If you are playing someone with a ping that is 80 or higher/lower than yours, you are going to experience a little bit of lag and who pulls first makes all the difference. If there is packet loss, (which seems to becoming more common these days and harder to detect) you are definitely going to have issues.
Packet loss is just like what it sounds like. The information "packets" that your computer sends to the game server gets "lost". While this is fairly infrequent, all it takes it the packet that is sending your firing data to "drop" (or get lost) and the guy you are shooting at has the advantage because he probably won't be dropping packets. Anyways. That's how that works in a nut shell.
that help a bit?