Jackal Personal Arm Shield
From Halopedia, the Halo wiki
Summary
Jackals use personal energy shields for protection from enemy fire. Worn on the forearm, they create a barrier that can deflect any solid projectile weapon except rockets, and grenades. It should be noted that grenades, even though deflected by the shield, can still explode and collapse the shield if detonated nearby.
Energy-based weapons can penetrate the shield and can even collapse the shield altogether. The MJOLNIR armor's shield was devised by reverse-engineering the Jackal shields, it was later improved upon it to make it much more effiecient.
In Halo 2, the shield appears to have been weakened. It is now possible to bring down a shield with projectile weaponry.
