EDIT: DONT have any effect
I dont have an advanced means to do a statistical test but after a lot of shooting side by side patterns on the walls I can see no effect all from mounting either a grip or an advanced grip.
-Both ADs and hip(I mostly tested ADS though)
-I tested on both balanced pattern guns and on guns with right or left recoil.
Its especially noticeable with the direction recoil guns that the patterns still move on the same vector.
Shumkaun on official forum made me wonder about this when he found it with the serpent and posted pics of the patterns.
http://forums.station.sony.com/ps2/i...parsion.79880/
It looks like the the fore grips arent simply of marginal use, they appear to beare a total waste.