On my recent quest to be tight, I've come across a chart for o ring sizes for the above gun. Instead of ordering kits, you might find them cheaper at specialist sites or buying what you need. Ive personally a fn 19 in .177 and are now going to look where to order certain rings from cheapest
In the chart, under reference, when your searching use bs infront
So example transfer port - bs802.
Also use a shore hardness which is higher as it will last longer (most are 70,80 or 90 - best)
Hope this might help somebody else if they Google or use this sites search facility.
Guide on strip down
http://gallery.myff.org/gallery/857558/STRIP+DOWN+GUIDE+FOR+A+FALCON+FN12.pdf
Im pretty sure I've the dvd here as well, if anybody wants a copy, pm me a email address and I'll try and send a copy over.