Kind of depends on what the threads look like after 20 years but a lock washer could help, a nyloc nut would also work. They make a reusable loctite too, purple I think.
With only one lock down it will never hold as tight as a set of good tactical rings but some times you just have to work with what you have.
Hi perf barrels and bolts
8 twist, 5R, Melonite, Wylde chamber