Carnimel
We have two cylindrical shells (with radii a and b and charges +Q and -Q) along the z-axis. There is a uniform magnetic field (in the positive z-direction) out to radius r from the z-axis. Because of the symmetry, the total field momentum is zero. Suppose we now connect the shells using a wire along the positive x-direction. The shells are slowly discharged. The magnetic force will move the shells along the negative y-direction (at least temporarily). If we stop the discharge shortly after that, the shells will have a momentum to the negative y-direction but the field momentum will point in the positive x-direction (due to the sliver of unbalanced momentum in the region to the positive y-direction). What did I miss?