Do you have an idea just how tiny 1 gram of gold is? It really isn't practical for bartering.
Yep, tiny, diameter is 8mm
It has a diameter of 8mm and a thickness of "yes". It's incredibly fragile, too. Doesn't make for a good bartering coin.
Probably a mistake to assume the USD value of something is relevant in a collapse situation. As you said ammo could end up being far more valuable that it currently is. Similarly Gold's utility as a way to transfer value could make it much more "valuable" in that situation.