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The class gun of the engineer is the sub-machinegun, which has already been covered in detail above. It is a good weapon at short to medium ranges but is difficult to use in long ranged combat. Besides the sub-machinegun, the Engineer has access to the standard all kit weapons. Of these the most appropriate choice is usually a shotgun. If you are sticking near armor you are going to encounter a lot of recon and enemy engineers who are trying to flank around any hit your armor from the rear. A shotgun will kill an enemy in one hit and helps greatly in these encounters.
In terms of anti-armor weapons, the engineer's main choice is between a selection of RPGs and a deployable attack like C4 or anti-tank mines. In my opinion RPGs tend to work better when you are on the attack and deployables work better when you are on the defense. The reasons for this are fairly obvious. Anti-tank mines and C4 require an enemy to approach them, which isn't going to happen if you're on the attack. When you are the defensive, however, it is more difficult to swing behind the enemy and attack from behind with your RPG, which makes the deployables are better choice.