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Lee case length guages aren't over accurate with their spec anyway. I had 3 .243 guages that were 15thou different in length. I just used the closest to spec and shortened the shortest for the 6mmCM.

Could be the Chambering or Max cartridge length instead of recommended trim length.

I use a .260 Lee case length Guage for my 6.5prc with a magnum shell holder. I tested a case and measured first before doing all cases.
Also I use a .243 Guage shortened down for my 6mm Creedmoor....