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Old December 9, 2018, 01:43 PM   #8
Join Date: March 4, 2005
Location: Ohio
Posts: 17,213

Is there any chance this is a highly compressed load? Often, those will push a bullet back out.

If that's not the case, then the only explanations that occur to me are:
  • The seating die's little stem is not in the same place each time (remove the top plug and check for debris).
  • You aren't getting the press ram all the way up each time (you'd be surprised how common this is).
  • You've got some wrong bullets mixed into the box or are using cheap bullets for which lots have been mixed.
I once measured a sample of 15 Sierra 150-grain .308 MatchKings each from the base of the bullet to the ogive point of contact with a comparator and got an extreme spread of 0.008", or ±0.004" from the mean and a standard deviation of about 0.0025". So I would not be surprised to see as much as about 0.010".

The thing to start with is what I did there: Use your comparator to measure each bullet base-to-ogive length and make sure they don't span more than about 0.010". If they are OK, one of the other factors is involved. If they are not OK, you've got an issue with your bullet supplier or with a gremlin.
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