Sizing die setting shoulder back but not sizing case mouth

Guthrie

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Just what the title says, rcbs rockchucker, lee 308 full length sizing die. The shoulders are being set back 2 thou but the case mouth diameter is stil .312 or more. What am i doing wrong?
 
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If the die worked previously as hardened steel, unlikely to have worn to the point it’s out of disc now. Since the sizing is on the OD only, the ID Is contingent on uniform wall thickness. OD-wall thickness=ID which leads me to suspect wall thinning may be the culprit here. Bit hard to measure with just calipers, a micrometer would be better but try to measure wall thickness at a few places on a couple different cases.
 

Guthrie

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If the die worked previously as hardened steel, unlikely to have worn to the point it’s out of disc now. Since the sizing is on the OD only, the ID Is contingent on uniform wall thickness. OD-wall thickness=ID which leads me to suspect wall thinning may be the culprit here. Bit hard to measure with just calipers, a micrometer would be better but try to measure wall thickness at a few places on a couple different cases.
These cases do have a couple thousandths neck turned off of them but i wouldnt think that would cause it not close the case mouth. Just to reiterate, its bumping the shoulder back but not closing the case mouth.
 
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