Winchester Primer Residue
I clean primer pockets religiously. Having said that, want to mention Winchester factory loaded handgun ammo uses a primer that leaves a primer residue which is very hard and tenaciously sticks to the primer pockets. Often find these primers in cases marked "Winchester" in the larger handgun calibers. These are definitely not the same primers that are sold to reloaders.
I used to spend time picking and scraping out the primer pockets with dental tools but have now found an easier way to get these primer pockets clean. Decap and soak the cases overnight in a cleaning solution made from half gallon water, dab of dish detergent, half cup of vinegar, and teaspoon of salt. Agitate the cases in the solution for better cleaning. In the morning, rack the cases for drying and save the cleaning solution which can be reused until it appears dirty. Once the cases are dry, the residue will flake out just like the residue found in Winchester primers sold to reloaders.
I like Winchester cases and like them much more now that I can clean the primer pockets easily.