Like many of you I always of preference like to start the game after leaving the Fallout 3 Vault with a Firelance and have now tried around 60 different locations so will sum up my worst and easiest locations . The worst that springs to mind, apart from the rocky sites that are always difficult, is the Super Duper Mart where on one trial the Gun landed on the rear of the roof. This then allows gun to fall to the waste land ground inside the shell. Dogmeat can get it if fallen on the roof but not when outside the game area.
The easiest was the barn r/o 101 where on one attempt the gun and 18 rnds of Alien Ammo all fell through the open roof and landed on the ground inside the barn with another 5 rnds between the barn and the silo leaving 1 rnd for Dogmeat.
Of note here is the way the ammo when falling on the Corvega can be seen to rock the car about or better still bounce high into the air and land at your feet.
If looking to the north sometimes two twin puffs of dark cloud can be seen in the air some 75 paces away at the start location of its appearance and if walking north without looking away they will stay in the air until overhead when another loud fall of heavy metal can again be heard all around.