I usually use a layer of blue Loctite between scope bases and the action, and it usually works pretty well. That's exactly how I attached the NULA rings (which today are Talley Lightweights) to the first NULA we had, a .270 Winchester that Eileen used for many years until she started getting recoil headaches. After working up a very accurate load with 130 Partitions, the rifle stayed sighted-in for a decade, despite being bounced around in float planes, saddle scabbards, and even a small boat in the Arctic Ocean. Just before hunting season, whether it was started in Montana after antelope or up North after caribou, she'd take one shot from the bench at 100 yards. It would land dead-center, two inches high, and we'd go hunting. The only reason the rifle eventually lost zero is the scope went bad.
If for some reason I want the bases to REALLY stay solid I use Acra-Glas Gel. (Don't think it's works any better than JB Weld, but I have plenty on hand.) Two rifles with Acra-Glassed the bases are my standard scope-test rifle, a Heym .300 Winchester Magnum, and a Mark X Mauser .375 H&H. The .375's bases got epoxied partly because the 98 Mauser action was never really designed for scope bases, and when the bridge is contoured for them there's not much thread-contact for the screws.
But yes, a lot of the time what we think is a scope failing to hold zero is actually movement of the scope-mounts, usually the bases--understandable because they're held on by pretty dinky screws, even when 8-40's. Firmly connecting the bases to the action can really help.
