Just thought I would share my recent experience in case it may help out some other fellow shooters...
I recently was in the process of developing my load for a current Rem 700 in .308. I was pretty happy with the grouping and was starting to feel confident to start shooting out further. After a fruitless hunting trip, I went back to the range for some more target time and was suprised to find my zero had shifted approx. 2 inches lower at 100m. I duly tweaked it back up but felt unsettled in my choice of a cheap scope.
Next time at the range, my zero was ok but the groups had really started to open up. I was starting to lose confidence in my rig and the felt I was best to ditch the scope and work up a new load. Subsequently, while waiting for a new scope, I re-bedded the action, had the barrel trimmed a couple of inches and a suppressor fitted.
When the new scope arrived, I was quickly off to the range with a batch of loads that I was sure were going to be driving bugholes. To my dismay, groups averaged around 2 inches with no loads really looking better than another. I was pretty frustrated at this point, and after changing so many things at once, was not sure where to start looking. Duly, I decided that the bedding must be to issue, so took to skim bedding the action again.
As I was in the process of cleaning up the bedding compound, I was applying pressure to the scope and felt the slightest amount of movement. I was sure it was all in my head, but after rocking on the scope I could quite clearly see that the scope base was rocking on the top of the receiver. Couldn't believe it. The one thing I hadn't checked , the scope base, and it had come loose!
Upon removing the one piece (B-Square) base, I noticed some raised edges on the underside of the mounting hole perimeter. When initially tightened down, the base had only snugged down against these raised edges. Over time, the edges had compressed, allowing the base to move. This is obviously a result of poor machining, but to remedy I was able to sand the 'lips' down flush and remount.
Back on the range, groups were back to well sub-moa, and I had learnt a handy lesson.
"The devil's in the details". I had a similar experience. Several years ago I bedded a base on my new custom. After the epoxy cured, I took the the base off, cleaned up, and mounted the base with a little loc tite.
For the next little while the rifle drove me mad. Turned out I had left tiny epoxy plugs in the bottom of the screw holes. I took everything off and put it back a couple times. The rifle would shoot a screamer group and then open up. I finally saw a faint mark on the bolt and figured the problem out.
I too had a base loosen up, now I jb weld them down, my be extreme, but it is one less thing to worry about. I have a habit of keeping my rifles, and I like rails.
They'll pop off with a torch and clean right up if ya ever need.
I like the Seekins rails with recoil lugs. I bed them and drill and tap for 8-40's. Install with loc-tite. I've only had the one problem. Luckily it was my rifle.
I had a base move that was very tight a few years back. I think it was a burris one piece. Many people don't realize how much tolerance there is in standard screw holes. I missed a nice buck because I was off five minutes.