I suspected that was generally the case.
You left out one more reason: .22 rimfire is “rimfire”, not “center fire” - a fifth primer configuration.
” ... You left out one more reason: .22 rimfire is “rimfire”, not “center fire” - a fifth primer configuration.”
Now it’s my turn, to apologize for insufficient clarity.
I was posting about centerfire primers. Their manufacture is almost a separate industry in itself - necessitated, in part, because priming compound is a primary explosive.
And the four most common US sizes are used - somewhat interchangeably - in all rifle and pistol cartridge cases. Shotshell primers are a fifth type, configured for shotshells (some very specialized adaptations do exist, to load shotshell primers into blank rounds and wax-bullet rounds for handguns).
I clean forgot a sixth primer size sold to private citizens: for 50 cal machinegun cartridges, and wildcat/proprietary rounds based on that case, like 408 CheyTac.
Small-volume sales continue, of Berdan primers for Euro and other cartridges, generally of collector interest. Had not heard of any recent US production; all are imported.
The primer of a rimfire cartridge is not manufactured as a separate component. Instead, the priming compound is mixed up in small batches (each still quite large compared to the priming charge for one RF case) and inserted after final rim shaping.