Poolman, welcome to GBO! Read the barrel fitting info in the FAQ and Help sticky, it may give you some ideas on what to look for as far as barrel to frame fit. If your .280 RMEF was made before '99 like my 35Whelen and one of my .25-06 Ultras, the frame is not as strong as anything made '99 and later, it's quite possible that loads too hot have been shot in it or the fit wasn't good to begin with. With the forend off, is the barrel loose on the frame? Is there any gap between the chamber and standing breech of the frame with the forend on or is the barrel loose side to side when the action is open? You shouldn't be able to see a gap(light) tween barrel and standing breech when looking across the chamber from the side, if there's gap there, there's a problem.
As had been mentioned on the sizing of brass, I check all brass in the chamber to make sure there isn't too much head space, the head should be flush with the chamber mouth ideally, if it's recessed any, add the gap tween chamber surface and the frame and that can be a problem. I set my dies up so the back of the case is flush to the chamber, then neck size only until I need to change it due to case growth and the shoulder changing, then full size just enough to make it flush again. I've even had a whole bag of new .280 brass that was too long by way too much to close the action to lockup. So don't assume new brass will fit perfect with checking it.
Hope this gives you some idea on what to check and helps ya get a handle on it.
Tim
