Hmmm, medium huh? lol. I cant wait to see -L 300 winmag maybe?
The L in XCR-L stands for Light, this is the one that currently exists.
The M in XCR-M stands for Medium, this is the one that might soon exist.
The H in XCR-H stands for Heavy, this is speculation.
Looks to me like the 300 Winchester Magnum is closer in size to the .308 than a .50_BMG (which is a guess for what the XCR-H will be), so I would guess that if there ever is going to be a conversion kit for 300 Winmag it'll be for the XCR-M?