Scoped the chamber and disassembled the dies; No debris in either and no other obvious flaws, so I suspect you all are correct in suggesting debris was likely the cause and it's likely it was dislodged during cleaning. I'll post back here after my next outing with a quick update just in case I...
Thanks for the input - given it, I'm reassured that this probably isn't something super common or a fault with my reloading technique.
Dents weren't there from the factory, so I'm leaning toward debris in the chamber or dies. I looked for pressure signs on the bases when I fired them but didnt...
Yes good eye... not sure what is up with the necks, I'm a reloading newb and prob failed to square them in my trimmer.
I thought the dent would be uniform in placement and angle if it was the die, but thinking about it maybe a loose brass shaving in the die? I'll check that, thanks!
This is once-fired Nosler .223 brass out of my Savage 10FP-SR. Full length resided once before loading. After tumbling them and before resizing the necks for another load, I noticed that out of 50 cases roughly half show this dent in various orientations on the shoulders...
The dents are...