Re: case mouth chamfer and runout
An uneven case mouth would have to be massive to cause visible runout.
Most run out is from poor necks. Nonconcentric or uneven thickness OR a poor sizer/expander making the necks crooked. No seater can correct a poor bent or uneven neck!
Perhaps the biggest cause of "bent" necks is a conventional ball expander that drifts to one side, due to neck variations, as it's withdrawn. I use a Lee universal decapper followed with Lyman M dies for expanding FL cases and use Lee Collet Neck Sisers for neck sized cases. I do have one body die and use the Lee die in combonation with it for "FL" sizeing for one very accurate rifle; love the effects of that combo.
From what I read on the net it seems many people think a high "bullet tension", a neck that's 3-4 thou under bullet diameter, is good. It is not. Such a hard forced fit into the case almost certainly produces signigicant run-out during seating. Seems the ideal "tension" is 1 to 1.5 thou.
A really good seater die (and that means a Forster BR or Redding Comp seater) will keep things straight but only if the necks are good.
One popular idea is to seat a bit, stop and rotate the caee and seat a bit more, etc, to reduce runout but that's virturally useless. Conventional seaters simpy aren't fitted tightly enough for rotating during seating to accomplish anythimg. If a bullet starts entry tilted it will usually continue that way. A proper neck tension helps them enter straight and the Lee or Lyman M dies assure that for me.
