Impact is knockback effect not damage, which is from the attack stat and as far as I have noticed has no effect in PvP. Attack is irrelevant anyway because weapon damage is all normalised for crucible. It is entirely likely that someone else got a tick of damage on them first, because from full health 2 handcannon headshots will NOT kill. It only does 86 damage for a headshot.

Attack stat does not do anything in crucible, it is all based on impact. Attack deals with damage vs armor (which is normalized in regular crucible), if you have more attack than the opponent's base armor, you do 100% of your impact damage. It's why your guns will do the same amount of damage to lvl1, lvl2, lvl3, lvl4 etc. enemies until you reach a point where one gets enough armor to start negating it. Impact is base damage, which means that crits (headshots) do that much more damage because they're a multiplier of the base damage. Mine do more than 86.

*shrugs* bungie is retarded then but thanks for explaining it.

I probably worded it bad, but some people made a discussion of it[url=http://www.reddit.com/r/DestinyTheGame/comments/28aj53/analysis_weapon_stats_attack_vs_impact/] here[/url] a while back, general consensus was that attack influences the damage towards armor (although exactly how isn't known) and that impact seems to be base damage, they have a couple of test examples on multi-level enemies too showing how the damage stayed the same until a certain point, and another one with low attack getting much higher headshot bonuses because of it's high impact. Since armor is disabled (outside of Iron Banner) the attack value seems not to matter at all in crucible, I even keep some bad attack stat weapons on hand for when I go into crucible if they have otherwise better stats than my higher level versions. A pity games don't come with detailed instruction manuals any more, explaining that (and a bunch of other stuff like elements vs shield types etc.) would have been nice to let people know.