Lots of interesting comments here. As Mule Deer has pointed out (not for the first time), there really hasn't been any truly new cartridge for quite some time.
Actually there are quite a few new cartridges designed to work in standard length AR15 (not AR10) actions. Some of these take liberty with thinning the part of the bolt that encloses the cartridge head (I have learned). Some use rebated rims.
Another new cartridge is the .300 Blackout or Whisper, for those desiring to use subsonic rifle cartridges with suppressors.
