Bagheera, Id start by deburring the inside of the case mouth - try to get a VLD deburring tool as it leaves a longer taper that helps the bullets enter much easier into the necks.
Secondly Id lube the necks with graphite powder.
Seating stem - like stated above - please ensure that your using a seating stem matched to the type of bullet your sesting.
Correct setup of the seating die, if the die is not set up correctly the case does not get the correct support and it too will cause the shoulders to buckle.
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