How do you get the fire lance alien blaster on fall out 3

I tripped over it, myself. I was wandering up the west map edge along a road that eventually led me above MDPL-05 Power Station and onward to Raven Rock or Mountain or whatever, up in the top left corner of the map (west of the river). I was sneaking (at a run) along the road, minding my own business, when I suddenly picked up a rad or two. Investigating that, I found the firelance and some ammo for it lying in a crack in the road. I found a few more ammo packs further north on the same road, making five in all. Unfortunately, my computer crashed a few minutes later, and I had been too stupid to save immediately after picking it up. When I retraced my steps up the same road, neither the firelance nor the ammo packs were there, so if you're lucky enough to find it, save *immediately*!