If I had to make rules for Riot shields they would be... depending on the type of shield.
If it is a Full shield, and not the GSG9 small shield, the rule would be that it cannot be modified to be able to stand up on its own, thus creating a "movable wall" allowing the user to use any weapon.
If it is a smaller shield, the user has to use a Pistol, or a small SMG (UZI/MAC/MP7/MP5K/NOT UMP) on single fire mode, (because in all realism, in real life, you cant fire a submachine gun on full auto in one hand with any hopes of hitting anything).
Last rule, no blindfiring, makes it harder
and gives other players more opertunity to hit you.