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ever 3-5 rounds I'd have one go off, make an 'off' sound and end up keyholing
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Lee Collet die- Light neck tension, low bullet pull. The firing of the primer is moving the bullet forward before the powder can build pressure to burn correctly. The 5.56 needs 45lbs bullet pull.
Size the neck correctly or buy a custon undersize mandrel from Lee. Lee info >
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Collet Mandrel
The Mandrels for the Collet Dies are made to .002" to .003" under nominal bullet diameter as there is about .001" to .002" of spring back. When the case neck is finished being sized, we want the case neck to be .001" under nominal bullet diameter
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This .001" neck tension is to light. The case neck should expand .002" after seating a bullet.