O.k first of two bugs with my ammo crafting: I bought a lot of ammo crafting stuff at Gunn runners (Jars of powder and primer boxes) then closed the dialogue menu and hit the pip-boy button, instant crash. I had a auto save from fast travelling there, confirmed it was the ammo crafting stuff by buying other things and opening my pip-boy.

number two: I finally got my ammo crafting stuff by saving after buying then exit/reloading the savegame. I tried to make .308 normal ammo and got -1176589 .308ammo from it, and sure enough my ammo counter at the bottom right displayed this as a number.

I'm fairly certain the first crash was to do with this negative ammo bug as when i looked at the quantity of one of the ammo ingrediants it was at -11million odd. Probably bugged the game out adding the large a quantity at once, but i thought it safer to report this.