Start the Training Mission and complete everything up to the grenade/tank range to access the other area. Go to where the jeep and truck are parked. Take the jeep (the truck wont start) and head towards the pool, go past a truck on your right then turn left and stop by the reception hut (instructor outside).
Go into the hut, as you look at the desk there is a switch on the right side, click it then continue to the pool and complete the objective, dont go back before doing the pool or mission will fail.
Take the jeep back and truck will now start, drive a little way out of the parking bay then stop. Climb up the bonnet/hood and onto the top of the truck then onto the roof behind, a gold bar is in the far corner.
Take it and complete the other objective (Explosives) then go back to the firing range, ignore the instructor and go into the the exit door to the fixed MG, the gold bar tirggers some extra shooting practice.