That sure looks like an incipient separation to me. I'd bet if you squeezed that case with a pair of pliers, it would break right at that line. What kind of brass is it, and how many times has it been reloaded? What load?

To answer your question, though, I have had a case separation in a 45/70. I was experimenting with fillers in the case, and had loaded a mid range charge of 2400 under the bullet, after filling the case with Cream of Wheat. Bad mistake. You can get away with fillers with black powder, but not, apparently, with smokeless.

It was purely my ignorance that caused it, I wasn't hurt, but there's a barely perceptable ring in the chamber of that rifle to this day.