I measure runout on every round. The 260 Lapua brass is pretty good, but not as good as the 308 that I shoot a lot of. With the 308 nearly every round is less than .001. With the 260 most range from .001 to .002 with some nearly .003, but none in excess of .003. I didn't notice compression effecting it any. What I did find is that with a micrometer seater you've got to dial nearly twice the required value when heavily compressing. With no other changes, the seater had to be turned down .010" with the compressed load in order to get the same seating depth.
That's one of the reasons I try to stay away from much powder compression.
Heavily compressed loads can create problems when seating bullets. Our Competition Seating Die is not a powder compression die. The excessive force required to seat a bullet on a compressed load can damage the die and may cause seating depth variations.
