The Juggernaut is not unlocked by default for players - it must be 'purchased' using [8000 XP]at the pre-game menu. It is decently powerful, with a moderate fire rate, but its main problem is the fact that it only has a maximum ammo amount of 60 rounds, which means that it will burn through ammo incredibly quickly, so single/burst firing is recommended.
Juggernaut has 10 ammo in the magazine and 40 spare ammo initially.
"The Juggernaut is an automatic shotgun worth worshipping for its omnipotent sustained firepower that's ideal for close range combat." — In Game Description