I looked even further into the shotgun ordeal. It was really bothering me.
The MEC does get the Saiga 12k semi-auto shotgun as the engineer. The US always gets a pump action, as well as the Chinese. The unlockable engineer shotgun is a <I>fully automatic</I> shotgun, however, called the Jackhammer. You hold down the trigger and it goes until it's empty.
The Saiga 12k is a really nice shotgun, but it's not fully automatic. It seems to have slightly better accuracy though.
-Lasereth