I was cleaning out my day pack, and found the 5 empty rounds of Heavi-Metal 16 gauge ammunition I used on my last few trips grouse hunting. I noticed something I hadn't seen when I pulled them out (non ejector gun, no excuse to leave 'em in the woods).

One has a nasty split down the side. ALL of them have a split that starts near the crimp.
I measured the chambers on the gun, and, near as I can tell, it falls into spec, including the .005 per inch of taper in the chamber. It is a Nitro Special, and I doubt much time was spent on the bores or chambers in this gun, not my slickest, by far.
The hulls themselves, seem incredibly cheesy. They are old, maybe ten years, they were also purchased from someone who bought them and never used them. Four of the hulls were fired in temps of around 5-10 degrees, one was fired in bluebird weather, perhaps 60 degrees.
No flattened primers, and these rounds were used in both chambers, and all have either a split, or, the beginning of one. Never seen the gun do this with any other ammunition. But, I haven't looked real hard before today, either.
Any thoughts?
Best,
Ted