I had the same problem with a nice old Savage in .250. I took it in to my gunsmith, who gently peened the circumference of the barrel shoulder where it contacts the receiver. He did it so carefully, I couldn't even tell what he did at first, and the bluing wasn't damaged at all. He just moved enough material to make it nice and snug, and I don't plan on taking it apart again, so it should stay solid for a long time. I've heard of people peening the threads, but I would think that the barrel shoulder would be a simpler method with less likelihood of causing too much damage. Depending on how loose your barrel is will depend on how much peening must be done, but in my experience with these rifles it will only be a thou or two.