I have had a look at the code, and if I've read & interpreted it all correctly, EMP weapons should have the following effects:
1) Any weapons (with hitpoints and without WIF2_NO_EMP_KILL flag) within 0.25 * EMP weapons outer radius should stop moving. It kinda looks permanent, I guess weapons auto-detonate when their lifetime runs out? (As an aside; the kill-within-distance doesn't match what the comments say, I guessing it should have maybe been 0.75 * outer radius. Regardless, IMO that hard-coded value would be a good candidate to add a weapons table entry for).
2) For ships that have these flags (SIF_BIG_SHIP or SIF_HUGE_SHIP), their turrets within the outer radius will stop firing while they are disrupted. The default time is 7.5 secs, this can be overridden by a weapons "$EMP Intensity:" IF the weapon flag "same emp time for capships" is also set. The EMP duration is also scaled as a proportion of the distance between the inner & outer radius. Ships can also have an "$EMP Resistance Modifier:" which also affects the time.
3) For other ships, it's pretty much the same except set of calculations/values except:
a) the flag "same emp time for capships" is irrelevant
b) EMP target loses their current target and reacquires targets randomly until the effect wears off
c) AI aim suffers a penalty (if I'm reading this right, it's a fairly big penalty)
Hope that helps everyone who had questions. If it's useful and you can verify these behaviours in game, it'd be good to update the wiki
