The M-421 is the standard weapon of the J-2A Juggernaut ADV it's a Steadygun type weapon that use a mechanical arm that force the user to always fire the weapon from the hip but this weapon have been made for this unusual configuration, it fires the new 9.7x68mm round that have excelent penetration and power still it have a more then decent controllability (also avabile in .338 caliber).
The weapon mounts a special folding gip and a side mounted pistol gip this allow a good control of the weapon from hip position.
the box mag can contain from 100 to 250 rounds depending on the version... the box mag in the largest version feature clips to attach the box mag on the hip or behind the S-7C shield.