In light of recent threads on cleaning primer pockets, I'll post this embarassing experience. I recently shot some .243 Win. loads that were slightly difficult to chamber. I left 3 rounds unfired from that box for evaluation; the rounds had been loaded in neck-sized cases. At home I got out the caliper and checked COAL, which was over SAAMI max., but there were no rifling marks on the jacketed bullets before or after covering the bullets with a Sharpie. Neck diameter of the loaded rounds was well below max. The length of the cases was within spec.
I then checked chambering of the unfired rounds again with the firing pin mechanism removed from the bolt; they were still slightly difficult to chamber. After checking chambering a few times I noticed the primer surfaces were burnished. When I neck sized the fired cases from that box, the primer pockets were heavily fouled. I had gotten out of the habit of cleaning primer pockets at every sizing. I won't do that again. After cleaning the primer pockets and seating new primers, the bolt closed like there wasn't a primed case in the chamber. Loaded rounds with cleaned primer pockets also chambered effortlessly with neck sized cases. Problem solved.
Cleaning primer pockets is not critical in many scenarios, but if you're loading with neck sized cases, or if the primers are not seating slightly below the case head, it pays to clean every time.