Marrakai:
Remember that I said 'sufficiently stabilized at the muzzle'. I think your experience with this rifle and the bullets you have tried are a good illustration of marginal stabilization - such that bullets of fairly similar weight perform quite differently. As a matter of curiosity, what is the rifling pitch of your barrel? And, remembering that bullet stabilization depends on bullet length also, how do the lengths of the two bullet types you mentioned differ?
mhb - Mike