By way of them not fitting yours but everyone else's, are they all chambered the same? Besides 5.56 rated at a higher pressure, the chambers are slightly different:



Also, depending on who made it will have a variance also:



On the brass itself, yes there are differences in thickness and annealing. One thing it could be is thin brass and not enough lube inside the case mouth so that the spindle is pulling the shoulders back with it.

I have a turret but also a single stage. I prefer to load bottle necks on the single though I can set the turret to single stage operation and do it as batches.

One thing that I do as habit with bottle necks is double stroke both the sizing of the shell and bullet seating. I'll send the shell up into the resizer, drop it down, rotate it a third to a half turn and send it back up. You could try that and it seems you may have inadvertently with your last post.

For the bullet seating, I'll set the bullet half way, rotate the shell as above and then fully seat it. I find it reduces runout.