Are you roll crimping or taper crimping? If taper crimping the case neck may be a little too large and hitting a small amount of carbon buildup in the chambers. If roll crimping, you can accidentally get too much crimp and the case end can swell out making seating fully difficult. If you aren't getting enough crimp the round may not be sealing enough causing blow by and causing carbon buildup.

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H Richard
SWCA1967 SWHF244