The riot shield is a lot of fun to screw around with in Ghosts, but it's difficult to use effectively. The truth of the matter is that its primary bash attack is not every effective and the maps are too big. Unless you sneak up on somebody or are in a very tight area you are going to lose most engagements with the weapon in you use it as a club. Still, it's a very effective situational tool especially when used with a team of players who work together. Here are a couple of builds and a strategy to use it.
Primary weapon: Riot Shield
Secondary Weapon: Any pistol
Attachment: Any attachment
Perks: Agility, Reflex, Strong Arm, Extra Tactical, Blast Shield
Lethal Equip: C4 or throwing knife
Tactical: Concussion grenade, Flash 9, or Thermite Grenade x 2
Streaks: Support, Ammo Crate, Balisic Vest, one other of choice
This is a very standard team support build utilizing the riot shield. It's meant to be used in a team game where you will be working with 1 or 2 other players in a group. The shields serves as mobile cover for you and gives the enemy something to shoot out why your comrades kill them from a few paces behind you. It also serves as a mobile platform for launching your equipment. Even though the animation moves the shield out of the way while you throw something, this is a glitch. You are still protected from the front while using equipment. The tactical options can stun your opponents and let your teammates or you kill them much easier. The lethal equipment gives you a ranged means to kill without sacrificing your protection. The throwing knife is reusable, but C4 is easier to kill with. Either way with both reflex and strong arm your equipment is very effective.
Perkwise this is a solid mix. Agility gives you a little more speed with the shield and reflex let's you get to your pistol instantly if you need to get a gun kill. Once you kill an opponent it's not a bad idea to swap your pistol for their weapon. Blast shield is here to cover you against opposing explosives in case your opponents have them. You could swap it out for something else if you want. The scrambler basically gives you the off the grid perk for free when you have the shield equipped. It also extends to any team mate who is in 20 meters of you freeing up their perk slots for other choices. The ammo crate will get you a primary weapon in place of your pistol and the ballistic vest will offer some additional health to you and your team. This is a build where you need to move as a unit and at a pace where the shield can protect you.
Primary weapon: Any primary
Attachments: Any attachments
Seconary Weapon: Riot Shield
Attachment: Portable Radar
Perks: Reflex, Overkill
This build is better for solo players who mostly want the shield to protect their back while using another primary weapon. While the shield is on your back it blocks all in coming fire and makes it impossible to head shot you from behind. Reflex can be useful if you want to be able to switch to the shield or back from it fast enough to stand a chance in a gun fight. However, you could drop it for something else. The rest of your build should center around your primary weapon and the play style suited for it. Equipment and other perks are optional but reflex will let you throw them faster.
The shield should remain on your back most of the time with this build. You can swap to it if you need the protection while sitting on an objective, or if you need to cross an open area covered by a distant sniper or sentery gun. You can also swap to it if you kill somebody and they have a dog kill streak coming at you. The dog cannot kill you through the shield and you can bash it to death pretty easily. Finally, the shield is useful as a scouting tool. If you are in a gun fight and your opponent flees, swap to the shield quickly. The radar will likely let you know if they are setting up an ambush or give you an idea where they went. You can also swap to scan for campers before you go around a corner.
Overall the shield fails as a blunt object. The knife just kills much faster. However, it makes one heck of a tactical piece of equipment.