A wire brush will not get the job done. The factory bead blast finish can be duplicated easily, with the correct beads. I don't see why you could not touch up a bead blasted gun.
When I bead blast a gun, I dissassemble the gun and remove the internals. I mask off the front and rear of the cylinder. I put the sideplate back on and plug the muzzel and the rear of the barrel. Comes out fine for me. Dean