Re: Head space ???
There are so many variables that you may not be able to tie any effects directly to a headspace difference of .003". Probably not enough to matter or be discernible.
IMO you will get more variance from a seating depth change of .010" from one shot to the other which will effect your velocity and thus your POI and group size.
I will postulate this, if your rifle is shooting 1" or less at 100 yds or 2" or less at 200 yards, there are not any small variations in case prep that will throw you out of a kill zone at 200 to 300 yards. IOW your group size may go to 5" at 300 yards if your velocity drops out of or climbs out of an accuracy node because of small variations, but that ain't a lost big game animal (same bullet, same charge weight, same primer, same powder not changed). If you are shooting a 3" group at 300 yards then a headspace difference of .003" will probably not make any difference.
But if you want to shoot a 3/4" group at 300 yards and not have it be a rare occurence, then you need to control all the variables.
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