I have a Kyosho fuel gun, and a mate has one of the RB guns.
They all leak a little. They both got a lot better after a few uses. I guess the orings just needed a little moisture to swell up a little and seal better. They still leak from the filler end as there is not a seal there other than the plunger. Its not a big drama unless you keep it full. I only put enough fuel in it for one or two refules. If more is needed my pit monkey can top the gun up from a bottle between stops.