Where does JB say the rifle was his? We don't know the reason NB purchased the rifle. Once a scope is sighted-in adjustments will need to be made by the shooter dependant on the target. We don't know whether the scope was usually on or off. The fact AP claimed he saw it on one w/e in July 1985 means squat. Why not ask others eg the housekeeper, Jean Boutell, or farm worker Len Foakes who were present inside and outside the farm several times a week. Even if it was usually on its hardly compelling evidence of anything?
If NB feared he was going to be shot I am surprised he didn't take more care with his firearms and store them in a proper gun cupboard.
In a police interview or statement, can't be bothered searching it out right now, but trust me.
The rifle?... a pre-Christmas present, as a substitute for the repeater shotgun which he queried the Eatons about and they pooh-poohed.
Once zeroed-in, only little adjustment is necessary by turning the focusing ring.
Even Nevill would find it hard to believe that his son was hatching a plan to shoot him, which is why he was tardy in storing any weapons away.