The only way to give yourself truly unlimited ammo is by enabling "God mode" with the command 'tgm'. This also means you will not take any damage and cannot die.

What you can also do is use the console command 'player.additem 000C1897 10000' to add 10,000 fusion cells to your inventory, and repeat whenever you run out of ammo.

There's also demigod mode (tdm) which is just tgm but less OP. Probably the most redundant console command in Fallout