using the riot sheild is fine, i dont know what level you are, but if you use overkill and equip a shotgun as secondary it works ok, if no overkill the FMG9 is a good secondary, even akimbo (insert pistol here) are good, i like the usp 45 low recoil and shoots fast, if your trigger finger is fast
the ARs work for some people, but im an smg guy, P90 is good, large mag size, but all the smgs are good especially silenced with range proficiency.
im gonna start messing around with the riot sheild alot more just because in game modes like team defender and dom its good to have a distraction like a riot sheild guy running around.
dont worry about the 0-15 thing, if you are using a riot sheild stay close to teammates, draw fire and let them get kills, you get xp for "crowd control", and if you die in the process they will grab your tags and hopefully kill the bastard who put you down, its teamwork and the riot sheild is not for massive kill games, i find it hard to even break even most games.
the riot sheild is the ultimate support role, take gunfire, get killed, defend objective, get killed, defend teammate, get killed, sheild bash one or two guys, get killed, while all this time your teammates are getting kills by killing the douchebags who are obsessed with shooting at you, the riot sheild is mobile cover for an organized team, not effective with randoms, because if you are not sheilding somebody you are not being a good riot sheilder!