The OP is using a Lee Loader and he has no control of inside neck diameter. And the Lee loader does not have the means to flare the case mouth.
I would like to know if the OP is using mixed brass that may have thicker necks.
The OP also used Sierra and Speer .311 bullets and crushed his case necks. And I seated the Sierra .311 bullets without any problem.
I'm leaning toward operator error and the lee die body being tilted during seating. The base the die body rests on for bullet seating is not recessed and it can be a juggling act to keep them aligned during seating. If the base moves off center the case will tilt in the die body.
Below my 1973 Lee Target Loader had a recessed base to hold the die body, and my .303 British Lee Loader does not. Meaning the older base would not let the die body move and kept the die body centered.
Below on the lower right the base is not recessed and can allow the die body to move. This also may be why the bullet runout was .007 and .009 on my seated bullets.
I would like to know if the OP is using mixed brass that may have thicker necks.
The OP also used Sierra and Speer .311 bullets and crushed his case necks. And I seated the Sierra .311 bullets without any problem.
I'm leaning toward operator error and the lee die body being tilted during seating. The base the die body rests on for bullet seating is not recessed and it can be a juggling act to keep them aligned during seating. If the base moves off center the case will tilt in the die body.
Below my 1973 Lee Target Loader had a recessed base to hold the die body, and my .303 British Lee Loader does not. Meaning the older base would not let the die body move and kept the die body centered.
Below on the lower right the base is not recessed and can allow the die body to move. This also may be why the bullet runout was .007 and .009 on my seated bullets.
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