Duraing a game of ATF Québec at Samuel Paintball 2 guys, Anne_Usse and his brother, made a Riot shield out of plywood with a plexiglass window. They used measurements of a real riot shield to build it. The advantages are quickly nulled by its disadvantages, the weight and size made it hard to maneuver easily so a team with a shield like that could still be anihilated and + it was a real bullet magnet since every body was shooting at it. So I think that no special rules should be implemented unless someone comes with a very light shield or built in such a way that the user(s) is untouchable, which I doubt is doable.
Teamwork is essential; it gives the enemy other people to shoot at. - Murphy's Law of combat
""There are no atheists in foxholes" isn't an argument against atheism, it's an argument against foxholes." - James Morrow