For those that asked. Here is a pic of a 6 Grendel next to a 243 case. It is kinda like a 6PPC set up for an AR-15.

243 uses 49 gns of powder under a 115 gn bullet to get to 2900 fps.
6 Grendel uses 29 gns of powder under a 105 gn bullet to get to 2700 fps.