John, et al,
While purple loctite is OK for certain screws on a gun, I don't recommend that any thread locker be used on scope rings.
This is a pet peeve with me, as I commercially mount hundreds of scopes each year and don't get them back with loose screws. The problem is that when you get a gun in the shop that has been loctited you end up buggering the screws to get things apart, and many times the screw has to be drilled out.
All that is needed for scope rings is a good fit (check with proper rod and adjust/lap as needed), a proper screwdriver or allen/Torx wrench, and proper torque -about 20 inch pounds. The rings need to be tighted like a head on a car - go round and round until the torque is equal on all screws. That's it!
There are too many gun mag writers who advocate the use of loctite in places where it is not needed. Some firearms manufactuers also use things like red loctite on no. 3 screws. I suppose they have their reasons, but if you don't know the things are loctited, it is easy to get a problem when you try to get things apart. If you know its there, then you can use a heat gun to loosen it, but I don't like to use that much heat near wood, paint or scopes.
Like I say, a pet peeve....
Scott