On this page of our guide to Sniper Elite 4 you will find out how to get to the magazine and how to destroy it as part of completing the third mission of the game - Regilino Viaduct. Also, we are warning you about enemies set up near the warehouse who may make it difficult to complete this optional mission objective.
The storage house is near the second checkpoint to the north (M3,4). The only thing that you have to do is blow it up. You do that by destroying the trucks ad killing the soldiers guarding it.
As you approach from the West, you have to sneak around three units stationed here. Watch out for the ones with the binoculars icon above - they can call for reinforcements using flares.
To sneak around this outpost, blow up the gas canister and the gas cylinder by the tents, which will distract the opponents. This should do for you to climb onto the hill and hide in the bushes by the ramp.
Here, it will not be easier. To eliminate opponents by throwing a stone at the car and wait for them to gather around it. Then, shoot at the gas tank. Of course, it is not too probable that you will kill all of them, but you should be able to handle the rest of them without any problems. The commander has a safe combination on him. The safe itself is in the ammo cache by the rails.