There are a few sections in the first few levels on the Doom 3 campaign where there will be 2 barrels sitting next to each other. The first location with 2 barrels will be as you are coming down the “Old Comm Engineering Elevator” and enter into “Old Comm Storage”. As you exit the elevator save the game then shoot the 2 barrels. Reload your save and repeat. Takes about 20 seconds each rep. Repeat till you have the achievement. If you don’t wanna wait till that section, about 5 minutes in you will see your first barrel. You could reload your save there if you wish. Otherwise it will take about 10-15 minutes to reach the storage room.