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Old September 8, 2012, 10:18 AM   #4
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Join Date: October 20, 2007
Location: Richardson, TX
Posts: 7,333
The rim is necessary for a rimfire cartridge.
+1; the rim allows the firing pin to crush the primer against a solid backing surface- the edge of the chamber.

The main design problem with a "rimless rimfire" cartridge would be ensuring reliable ignition without piercing the case or weakening it so much that it comes apart during extraction. The design would have to incorporate some sort of device to support the "rim" as the firing pin strikes it, yet doesn't interfere with extraction.
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