As Joe said, it's all about the pellets. The FWB is capable of shooting amazingly accurate. Just this morning I went to my arms dealer (Krale) to test pellets, and found out which pellets my Walther LG210 likes best. Turned out to be the RWS R10 Match with 4.49mm head size. Not the cheapest, but as a match shooter you need the best. This batch came out amazingly accurate, so I immediately bought 5000 of them.
20 shots through one single hole at 10 meters, with diopter sights. I'm going to do some testing to find out how my FWB300S likes these.
Edit: the FWB300S doesn't like the RWB R10 Match pellets, but that makes sense since the older FWB rifles like softer lead pellets, like the JSB's (and Air Arms, which in fact are made by JSB). Grouping with 10 pellets was 3 times bigger compared to the Walther 20-shot group.