jorj82 - prudent call, but I don't remember if dynamite worked in this instance. Won't hurt to try.
godfather - I don't remember any way of getting the howitzer shell other than opening that ammo crate, but if you feel like cheating a little, [spoiler:7a9b9b90fb]you can try pressing '2' on the keyboard when you are entering from the world map, on the "town map" screen (the one where you can select the green "Battlefield") arrow - keyboard number keys are shortcuts to "unrevealed" map locations, which should get you inside.[/spoiler:7a9b9b90fb]