To do this you must first find some alien power cells (from Firelance, by Fort Bannister, in Fort Independence, or at the crashed ship).

Then you have to go talk to Protector Casdin at Fort Independence.

You can give him alien power cells in packs of 12.

Give him lots but make sure to keep some.

Then reverse pickpocket him the rest of your alien ammo. Don't exit the pickpocket screen.

Now you can take it back because the game thinks all of the ammo is yours. Repeat as much as desired.