I did a little catch up reading last nite, and a few different scenarios came to mind, and I thought I would share. Given the fact, that this and all belted mag cartridges HEADSPACE on the BELT, and not the shoulder. ( at least when it is new, and unfired). That being said, I have measured quite a few pieces of brass, as well as loaded factory ammo, that vary as much as .035-.040 from the positioning of the shoulder on the brass, and the distance ot has to travel to rest up against the chamber. It seems the best brass and ammo I have seen is the Winchester. So, as soon as it is fired, all the momentum and pressure move forward, and the weakest part of the brass right ahead of the belt now has a thin spot. So now when you resize, whether it be full length, or neck size, the shoulder is pushed back, and the ring above the belt starts getting stressed, and brittle. When you throw in the possibility of liberal chamber, and very soft brass, ( Federal ), you have a recipe for incipient case head separation. Even though neck sizing has been used, it is posible, ( probable), that the neck die is bottoming on the top edge of the shoulder, and STILL pushing the shoulder back, and again, stressing, spelled " kinking", the weak spot.
Dies need to be set up and adjusted on the very first firing of a single piece of brass, both neck and full length. Another thought, is to have a chamber mold taken, and check it against SAAMI specs.
Additionally, even if you do have a liberal chamber, a long seated bullet on the very first firing of new brass will keep the brass from stretching, then if the dies are set up properly at that point, brass life, and safety will greatly be enhanced.
The problem here is not unique to the STW, but to any belted mag. Once those dies are set, they are unique to that gun only. So for those that have multi guns, in the same caliber, it is better to get another set for the other guns, because all else is NOT equal. This has worked for me, as I have fooled with belted mags for many years, and have had my share of thinning case walls, but I've never had a rupture. That can destroy a chamber REAL quick. The escaping gasses can actually cut into the chamber, and create a bigger problem. This is just my ( 2 cents ), and IMHO, should be at least considered, but those pix are scary, and are screaming, something is VERY wrong here!