Vic Firth Ian Grom Corpsmaster Medium Hard Marimba Mallets feature a rubber core wrapped in synthetic yarn. They're made to be warm and blended in a front ensemble environment, while able to handle the harsh climates and velocities of marching activities.
These mallets are made with 5/16"-thick birch shafts that counterbalance four mallet grips nicely, with a 16.75" length to reach wide intervals.
The M263 Medium Hard models work best above middle C, with just enough bite to sound at the highest ranges of the instrument. These will be useful in the right hand often.