Go to Bethesda ruins, kill all the raiders, then go to the wrecked bus in the middle of the Bethesda ruins.

Save the game, then walk on the road to the truck near the rocket petrol station.

It should happen, if it doesn't, reload the game and do the same thing again until it does.

Then find the ammo and firelance, you now have the second most powerful weapon in the game (the best is the experimental M.I.R.V).