I don't see it that way. If the chamber is "long to the shoulder" than the brass will stretch and thin ahead of the belt.
But he said his cases looked normal with no signs of pressure. If they were stretching to the point of failure him and his experts, whoever they are would have noticed.
Exactly. And, the brass (instead of stretching) may well have just bumped the shoulder forward as is done in forming something like the .300Wby from .300H&H, or similar.
Whether the shoulder bumps forward or the case stretches just ahead of the belt depends in large part on the headspace as set by the reamer, and to a lesser extent on how hard/soft the brass is. If the head space is too long, the case stretches above the belt in part because the WM is fairly straight walled round which under pressure helps prevent the case from sliding rearward. That's why when you blow out cartridges (like the JDJ rounds) you seat the bullet into the lands so the case head is firmly against the breech forcing the majority of the stretching to occur at the shoulder. Excessive headspace in an Encore can cause problems, but it's unclear how it would have caused the OP's issues as blown cases don't cause the pistol grip to break.
