Phalanx Shield is a tier 3 skill in Axton's Survival skill tree. Your Sabre Turret projects a shield that attempts to block enemy ranged fire. Friendly ranged attacks pass through.
Turret Shield: +1
Enemies can still move in or out of the shield without being blocked. Enemies that attack the shielded turret with a leap will first impact the shield and deal damage to it, then fall into the shield without doing damage to anything inside.
The Phalanx Shield can only take a set amount of damage before it shatters. The amount of damage a shield can sustain is currently unknown, the only known factor affecting it being the Commando's level.
When using Gemini, the dual turrets each have their own separate shields, offering protection to two areas. Vault Hunters can stand inside overlapping shields for double the protection.
While the Phalanx shield can only absorb a limited amount of direct damage, it seems to be very effective when protecting anyone within its range from AoE attacks that would inflict a status effect (e.g.: burn damage). This effect is particularly notable during the final battle with The Warrior, specifically guarding against his fire breath attack and completely negating any effect/damage that would otherwise be received while exposed. As long as the the Phalanx Shield does not take a direct hit, it is capable of surviving for the entire duration of the Sabre Turret's deployment.
The shield can be used to block Hyperius the Invincible's nova's by dropping it down right before it goes off and hiding inside. Once the nova is over the turret can be reclaimed and the short cooldown will keep it available for the next nova.