Nope the .243 Win won't fit in the mouse frame.

For more range in factory loaded rounds...
6.5 Grendel
.243 WSSM (spendy) He also has a 6mm DTI that is a 6mm x 6.8 SPC
http://www.dtechsuperstore.com/index.html

If you handload there is a 6mm x Grendel called 6mmAR
http://www.6mmar.com/
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NO STEP ON SNEK