I have a 3x and a 4x - both in the universal fitting.
I use the 3x on the back of my Leopold 6-20 EFR and absolutely love it.
I found the 4 times didn't allow the ocular bell to be adjusted in enough on the Leupold to allow the crosshairs to come into sharp focus.
Warning - they will reduce your eye relief to less than half of normal.
But I found that is a good thing because it required the scope to be moved back thus enhancing balance.
There is a very slight loss of resolution, but barely noticeable with a good quality scope such as a Leupold.
I don't know how to measure it exactly, but it is rated to add around 30% more magnification to any scope.
3x's = plus 30%. 4x's = plus 40%, so the literature advises.
I recommend the 3x's highly.
EV2 Mk2 Leupold 6.5-20 EFR
AA S400 .22 FAC SL Special Simmonds 6-18
Last edited by MAX-nz; 19th February 2010 at 08:13 AM.
Reason: bad gramma