is this from a vice or some shooting position where your actually touching the weapon at a point other than the trigger? My drill sergeant used to say to tighten up the nut behind the but plate
Basically unless I eliminate my own (shooter) error, I assume its my fault if there is a group irregularity.
If these groups were shot at 200 yards or more, I'd try a different ammunition to see if there are large velocity variations. This would be inherently poor accuracy ammo in any rifle. A chronograph could verify any possible velocity variations.
It's more likely that the ammo is poorly suited to the RFB, and the bullet is exiting the muzzle when the barrel harmonics has the muzzle moving at the greatest speed at the center of the muzzle deflection, rather than at the end of the muzzle deflection when the movement is the least. Imagine a pendulum swinging, where it pauses at each end of the swing before reversing directions and is moving fastest at the middle of the swing. The cure for this is also different ammo, although this ammo could be accurate in another rifle but not your RFB.
The other likelihood is some mechanical problem with the scope, scope mount or rail that allows vertical movement. That problem would persist with different ammo. This may be the most likely scenario, particularly if you're seeing something like 5" or more of vertical stringing at 100 yards.
Check your rail attachment screws, especially the front one. As mentioned, the shortness of the RFB makes it a little less stable than a longer rifle with it's longer moments(barrel and stock), but I doubt that would cause consistent stringing. The rail attach point is a weak link in this design. I wish they would have made it a little bigger and used 2 screws, or one larger one...
This was just discussed here a week or so back. I posted the torque specs for that screw and the other recomendations I received from KT in that thread from when mine came loose...