BUL Loaders are Loaders that are a little larger than normal ones, and they also have a large shield attached to their left arm, deflecting any bullets fired at it. They will slowly walk toward the player, firing at them. The arm can be shot around in a lot of cases however so be sure to watch the position of the arm. If you manage to shoot their arm with the gun off, they will usually try to fire the eye laser at you, leaving them a lot more wide open.
If the arm starts to lower to the ground, the BUL Loader is going to charge at you. You can dodge to the side or jump over them, whichever you prefer. It is important to note that the arm can be easily shot off, especially in co-op, so it should be a priority if you have access to the side of the arm.