Barter is always based on the needs of the parties. Let's say I'm a hungry old man with a stockpile of .22LR ammo. A young man with a rifle may be willing to trade me rabbits for ammo. How many rounds of ammo is a rabbit worth? Certainly more than one, since it takes one to shoot the rabbit. Two would allow the hunter to shoot one rabbit for me and one for his family, but wouldn't allow for misses or practice. We'll eventually agree on an exchange rate that works for both of us. If there is more than one hunter or more than one ammo trader in town we'll have to be competitive. Neither of us will have any real need for silver unless and until it becomes accepted as money in our local economy, as it probably will as time goes on.
(The moral of the story is no matter how much silver you have, always have a pile of ammo, too.)