I had a customer bring in a Virgin Valley 300 WSM
Encore and he was having problems with case head seperations just above the case web and always on the top of the chambered case.
I knew exactly what it was because I had Virgin Valley build me an Encore many years ago chambered in 7.21 Tomahawk from Lazzeroni. Basically the same thing as a 7mm WSM
but a bit shorter and a bit fatter yet.
My tomahawk was scary accurate but did the exact same thing, severe case stretching on the case head at the top of the chambered case.
The reason, excessive frame flexing in the Encore. The 300 WSM the customer had brought had been shot enough to have permenantly damaged the frame and it had to be replaced.
In my opinion, from shooting the Encores ALOT. The Belted Magnum case head diameter should be considered max with modern high pressure loadings and I much prefer to see the '06 case head size used in this design.
I know there are chamberings for the 416 Rigby, even factory offerings I believe but these are designed to be loaded to very low pressures.
Even then, I had severe stretching with my Tomahawk.
My recommendation, if you want 300 WSM levels of performance in the Encore, take a hard look at the 30-06 AI. In a properly dimensioned chamber designed for modern '06 case dimensions, you will match that the 300 WSM will get with the 30-06 AI chambering and it is MUCH better suited to the Encore design.
Just what I have seen playing the game with the Encore. Great rifle but you must stay within the limits of the design and I recommend staying well within them for best performance.