R6:S

>Are man-portable APS even possible with our current tech?
Yes in theory, but hard kill countermeasures are of limited usefulness since they pose as much risk to the users as to the projectile.
>And how retarded/good is the idea of rifle-mounted gun shields?
Super retarded. Anything that will stop rounds greater than pistol will weight several kilos and restrict your view, which is more a liability than a boon. Theres a reason people who use ballistic shields generally either use a pistol or nothing at all.