after the tanks are destroyed and the infantry have been shot you can shoot and remaining
morter men with your gun

There should be a few Springfield sniper rifles around, so you can take out the mortar
crew with them. You can tell where they are by the smoke from the mortar after it fires.

Keep in mind, about five seconds after you destroy all the mortar men, an explosion
destroys everything there, including your guns, so if you want your gun back (that you
switched for the Springfield), you better hurry!