There seem to be 2 schools of thought on this one.
I used to use JSB heavy weights through my Mk3 and they grouped ok but in the wind I seemed to give around half the windage of other people I shot round with.
On the down side the aim point between 35yards and 45yards was around 1 to 2 inches diffrence due to dropping off at range.
I have swopped over to mozzies as the heavies seem difficult to get hold of, they group better but do seem to move alittle more in a breeze
I'd rather a full bottle in front of me than a full frontal labottomy