As said above, try out around the 4.0mm front sight size for starters. I seem to recall that the FWB 300 series had a slightly different front size element on them (ear size). If you try the link below, I am sure they can accomodate you
http://www.edinkillie.co.uk/ecatalog/index.php
If you look there too, you can see some of the exotic stuff available. I am sure the good shooters can spot the difference, but fo rme, it is far above my ability
HTH
ATB
Phil
AAS200T.177 & S510SL .177. FWB P70(diopter), P70 FT(Big Nikko) Steyr LP10, LG110(Big Nikko), LG20(diopter) . The gun is deadly accurate - shame about the user