8" is a bit much IMHO of swing to put it on not swabbing between shots. Also given the fact your routine prior was to clean every 5, one would assume you came up with that by trial and error. Given your mention of the other guns accuracy I don't think shooter is to blame here either, unless fatigue may have been a factor. It is possible to have sabot failure by to tight (fouling) or loading when hot and grooving. I have seen some weird results due to sabot related problems in both sub shooting and smokeless shooting ML's.

A friend of mine had a mount come loose on his 300wm recently and it pooched his 4200 elite...so that is certainly a possiblity. It also may be your mount or ring is damaged as a result as well. I would inspect them closely and if they appear GTG try a verified scope to see if the problem goes bye bye.